Should the square footage of an ADU be combined with the square footage of the main house? This question has been coming up, and I wanted to give my take that there are some solid reasons why the ADU should NOT be combined with the primary dwelling. What do you think? Let’s talk about it.
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10/2/26 PCAR Rocklin
10/21/26 Coldwell Banker Sierra Oaks / EDH
11/19/26 Appraiser webinar TBA
NOTE: Our MLS system has an ADU category so agents can list the square footage of the main house and ADU separately. Not every MLS system has this.
REASONS WHY THE ADU & MAIN HOUSE SHOULD NOT BE COMBINED
1) ANSI measuring standards are used by appraisers (and Fannie Mae)
As a rule of thumb, appraisers don’t combine the square footage of a main house and a detached ADU due to ANSI measuring standards not combining them. Living area must be contiguous, have heated space, at least 7 ft of ceiling height for over 50% of the dwelling, etc… For reference, Fannie Mae requires appraisers to use ANSI, and this is a huge point since ANSI measuring is now standard practice for conventional financing (the most dominant form of financing). Look, I don’t think Fannie Mae has absolute power to create the rules in real estate, but loans are literally happening on listings based on these rules, so it’s key to understand how Fannie Mae and appraisers are interpreting square footage and thinking about comps too (looking for homes with an ADU rather than a larger “comp” that has the combined square footage). If I had to guess, I would think FHA and VA are going to follow Fannie Mae’s lead here in adopting ANSI, but I could be completely wrong.
By the way, I’m talking about a detached ADU. If the “ADU” is within the footprint of the main house and there isn’t any expectation of privacy, it’s reasonable to include that in the square footage of the house. In cases like this, the space is probably already listed within the square footage from the builder. On that note, for agent friends, be careful about listing this as an ADU in MLS because it’s really just a house with a cool ADU-like layout. I’ve seen this mistake where agents will say a home is 2,700 sq ft with an additional 500 sq ft ADU, but the “ADU” part of the layout is already accounted for within the 2,700 sq ft.
2) It can count in the value even if it’s not in the square footage
Just because it’s not in the square footage does not mean it can’t count in the value. I realize some people get frustrated that the ADU doesn’t add to the square footage, but this doesn’t mean it can’t be accounted for in the opinion of value. Some people say the owner could lose value if it’s not combined in the square footage, but could an owner lose value if it’s combined? What if a home with an ADU could command a higher price than the combined square footage size? What do the comps show?
3) It’s fundamentally NOT a larger house
As a non-laywer and non-broker, I wonder if it’s a liability to lump all the square footage into one number. If you have a 1,600 sq ft house and a 400 sq ft ADU, is it really a 2,000 sq ft house? No, this is a 1,600 sq ft home with something else in the backyard. Or if you have a 1,000 sq ft house with a 900 sq ft ADU, are we looking at a 1,900 sq ft home? Nope. MLS Rule 7.12 states, “all data entered in the required fields on the multiple listing service must be accurate and meaningful.” I guess I question how the combined number could be understood as “accurate” per MLS rules when the field is supposed to represent the primary square footage. All that said, I understand when agents tell me it’s a marketing disadvantage when having to say 1,600 instead of 2,000. Yet, is the combined number misleading? If so, is that the trump card here?
4) It makes it easier to pull comps
When pulling comps, it’s so much easier when we have accurate information. It’s very helpful to know the exact size of the main house and accessory dwelling when trying to figure out value. This is especially true since some ADUs are tiny, while others are 1,200 sq ft. Our MLS has an ADU category that makes it easier to search for comps with ADUs, so combining square footage here seems like a step backward rather than helping us pull comps better. I wrote about finding ADU comps here. By the way, the combined value of a house and ADU may not be the same as other larger homes. What do the comps say? Lastly, I find some want to apply the price per sq ft of the main house to the ADU, but be careful of simple ADU math like that.
5) Let’s strive for better data
We have the ability today to segment the size of the main house and an accessory dwelling, so it seems fitting to do so. Doesn’t it seem like progress to have better data? Granted, I realize not every MLS system has this feature, but ours does, and it seems prudent to be able to break data down further so we can be more accurate. I also realize it’s tough when online portals only display one square footage category.
6) Segmenting ADU square footage is a trend
California is leading the country in ADU permits, so having accessory dwellings show up on MLS is only going to become more common. Thus, let’s be sure to reflect the presence of ADUs as best as we can. Even Realist locally is starting to segment the square footage of an ADU separately from the main house. In short, it seems like we have a trend here of big data trying to get more specific, and I think we need to lean into that. Granted, I realize not all online portals are going to easily segment size here, and that’s a problem to be worked on ahead.
7) Marketing can be challenging, but this is where agents shine
I find some agents say it’s more challenging to market a home with an ADU when the square footage isn’t combined because consumers aren’t readily seeing all the space. I’ve also heard that people wonder why the price per sq ft is so high for a smaller house. Look, I’m not on the selling side, and I don’t pretend to know the frustration here, but agents are masters of marketing, and their marketing skills can overcome this issue in my opinion. I’m not saying the MLS system is perfect here either, but I strongly agree with segmenting this data for the sake of accuracy. When it comes to price per square foot, that can change for so many reasons too. If a home has significant upgrades or a sweeping view, for example, that can dramatically change the price per square foot of a property compared to the comps. I don’t mean to downplay the frustration here, but segmenting this data helps get us to more accuracy while avoiding being misleading. And yes, I do think it’s a hurdle to market properties like this, but this is where agents can work their magic in marketing.
CLOSING QUESTIONS
Is there room for other opinions?
Of course. Let’s talk about it.
Why are you micro-managing agents?
That’s not the goal. Communicating accurately helps all of us when pulling comps and trying to figure out value. I want the ideal here, and I also realize it’s tough sometimes to fit a square property in a round hole when it comes to communicating on MLS and portals.
Are buyers and agents seeing these properties?
I hear the argument that consumers aren’t seeing some homes where the ADU is not combined in the square footage. Maybe that’s right. I guess the opposite could be true though where consumers might not see a smaller primary dwelling if an ADU is combined into the square footage. I guess this is a good reminder to maybe look at different sizes of homes in a neighborhood just in case there is something a little different that might work for a buyer.
What if my MLS system doesn’t have an ADU category?
This is a struggle, so there may not be a clean way to quantify the square footage of the ADU. I think this is why people sometimes combine the square footage and then clarify in the property description. Look, I’m not the MLS police here, and I’m not a broker, so I won’t tell anyone what to do. I will say that I’m not a fan of combining the square footage as it doesn’t communicate with the type of accuracy that is most helpful. Ultimately, it seems like the solution is to have a separate ADU category in all MLS systems so we can have better data and clarify how square footage is broken down across various units instead of lumping everything into the primary unit square footage. Some people suggest some sort of a total square footage category that includes everything beyond the primary dwelling. I wouldn’t have a problem with that, but I think most buyers are shopping based on the square footage of the primary house. In truth, there probably isn’t a simple way to perfectly quantify this, and even local agents aren’t always happy with the way our MLS is set up because some want to combine the square footage. I guess the big truth here is not everyone is going to be happy, and when adding more units, communication is going to get more complicated.
Can’t we just combine the square footage like a multi-unit property?
Some people say a home with an ADU is now a multi-unit property, and I get the sentiment, but ANSI measuring does NOT apply to 2-4 unit properties, and that’s why a duplex is going to have the combined square footage listed.
Is a single-family home with an ADU a duplex?
Some say a single-family home and an ADU is now a duplex, but the permits say the extra unit is an ADU, so there is a distinction here. Maybe the market could view it as a duplex, but remember that a duplex is an investment property, and in so many cases a home with an ADU might not be an investment at all since many use it for owner occupancy and extended family (no unit is rented). An ADU tends to be inferior in most ways to the main dwelling also, whereas a duplex tends to have units of similar quality in most cases. All that said, I think the line is getting blurry between an ADU and a full-fledged unit, and there is going to be a learning curve ahead for the market in this regard.
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Question: Do you think an ADU should be separate or combined with the square footage of the main house?
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Thank you for this post! So many good points!
Thanks so much, Shannon. I appreciate it.
Thanks for spreading the word on how to enter ADUs into Metrolist. It’s a pain to pull out properties that have been entered incorrectly.
And then you have the “2 kitchen” connundrum. Nice to know that MLS’s are catching up!
Why stop at two? Let’s go 3 or 4.
Sure thing. It came up again this week. I feel like I’ve written this post every other year over the past decade, but there is always some new layer to discuss.
The key here is “accessory.” This is not the primary dwelling unit. Many ADUs are merely converted garages – and mostly not of comparable quality to the primary unit and not of equal value per square foot in any case. They’re two different beasts and should be treated as such.
Yeah, that’s a good way to think about it. It’s “accessory” to the square footage AND the value of the primary dwelling. I think many owners want to instantly net the cost, but that’s not always possible. Getting dollar for dollar in real estate can be a real challenge with so many improvements.
In New Zealand, square footage listing (“sq meters”) includes the garage 🙂
Ha. Thanks Josh. And it includes the garage? That’s wild. Does it break down the size difference between the garage and the house at least? Hopefully so.
Hi Ryan,
I have run into a similar situation several times, most recently yesterday. A client has a 1547 sf home, he added 300sf of a fully insulated sun room attached to the house with heating and air and he asked me if I would market it as an 1847 sf home. My response was that could mislead buyers and our MLS rules prohibit that. They will probable find someone else to list it who will add the footage. My other argument to them was that appraisers are not going to give a sunroom the same value per square foot as the house itself. What ANSI rules apply here?
Hi George. Thank you so much. I always appreciate you chiming in. Here’s my question when people ask me about sun rooms. Was it permitted? Yes. What was it permitted as? A Sun Room. Okay, so it wasn’t permitted as square footage then, right? Okay, so then we’re not going to consider it square footage since this is permitted and built as a Sun Room. I’m sure we can bring in ANSI here somehow, but sometimes just talking about permits does the job. Moreover, sometimes it’s a tell with the language used. What I mean is, if someone calls it a Sun Room, that might mean it’s something else other than square footage. I’m guessing there is a profound difference in quality here too because it’s built as an enclosed patio basically – not living area.
How has AB1033, the recent legislation allowing property owners to sell an ADU as a condo affected values in SF and San Jose? Has there been any thought how it will be dealt with when it comes to Sacramento? Will the combined value of the SFR and the Condo represent comparable value in surrounding properties? Just another twist to consider.
Hi Tom. This is such an interesting law. While it is state law, each county has to adopt the law. I don’t think Sacramento has adopted this yet, so we’re not going to see any local examples until that happens. My observation is a law’s existence doesn’t always mean we’ll see ample market activity. I’m guessing there are not too many examples yet in other counties, and I just saw an article talking about the first one in California a few days ago in San Jose. In short, we’re at the beginning of this. I can’t even call it a trend yet either if there is only one sale so far. It’s a weird dynamic though because the SFR as well as the ADU become condos under this new law, so technically speaking, I suppose we would appraise the ADU unit in the rear on a condo form instead of considering it as an ADU. It’s strange to me because the permits say it was built as an ADU, but then we start referring to it as a condo and maybe look beyond the ADU. This is a challenging situation for comps also because we’re unlikely to find many truly comparable homes until we have more sales like this. I do wonder how many owners will convert their parcels to condos. I suppose if they were going to sell, then maybe there is an advantage since the sum of the parts could be greater than the whole maybe. But if this is an owner-occupied situation, I’m still wondering how many owners will do something like this. It just doesn’t seem like there is much incentive unless trying to help a family member afford housing or maybe needing a little money while not selling. Converting to a condo is a big change. I’m eager to see how this unfolds.
curious on selling the ADU as a condo, what does that do to the main house’s privacy and the impact on the main house, not to mention a smaller lot size and does the condo have to have an ingress/egress easement, then of course that leads to the who maintains the easement and does that also impact the usable site value and cost to the maintenance of either or both properties that maybe another sale doesn’t have. I have a comment about Sacramento Government stuff, not the folks living there, but will refrain. Next you speak of ANSI, prior to ANSI the lenders / Fannie and Freddie and VA, FHA did not allow a guest unit or ADU or Casita to be added into the measurement of the main house from an appraiser. This isn’t a new item. P.S. AMS (American Measurement Standards) which is what I use on my private work. ((don’t get me started on ANSI and measuring stairs twice and other XX>>>S??!!!###$$$$%%@@@! whew caught myself.))
AMS does not include the ADU in the main house.
Ryan things have been way to interesting the last 4 weeks bet I missed a few other fun posts by the all-knowing, and all seeing, Ryan Lundquist. Thank you, my friend, always good stuff you post. Hope you are feeling good and avoiding Starbucks unless for tea.
I appreciate the commentary. I wish I would have mentioned that about AMS too. Great point. The stairs thing drives me nuts with ANSI. I don’t understand how we count the stairs and then the area under the stairs. C’mon!! That doesn’t make sense to me, and I would write the rules differently if I was in a position to do so. Otherwise, I don’t have any real issue as it mostly seems straightforward.
I’m avoiding all coffee. Only tea for me. That’s my motto in life. I hope you are okay, Brad. Not sure what “way too interesting in the last 4 weeks” means.
I strongly feel that the ADU should be listed as separate sf. The MLS should have a place to list it separately. Then there’s no question.
Thank you, Anne. I agree with you. I’m very thankful our MLS does have this category, though it sounds like not many have this across the country. I suspect in coming years we’re going to see more standardization about this, and there will be a learning curve. It should help move things along that Fannie Mae has adopted ANSI measuring standards. Game over if FHA and VA adopt also (okay, that sounds a little hyperbolic, but I think it helps to have this).
Thank you. . . Thank you. . . Thank you for explaining the correct way to report and market SFRs with an ADU. They are not duplexes and the living areas and bedrooms/baths should be reported separately in MLS. I’m curious to learn how the new 3.6 platform and the GSEs are going to handle an SFR with multiple ADUs, or a duplex/triplex like I am seeing more of now. Will it be classified as an SFR, or will it be a multi-unit income property?
Right on. Thanks, Richard. You know, the GSEs are just starting to bend toward accepting ADUs on multi-unit properties, so I think we’re at the beginning. I’ll have to talk more about 3.6 at some point. I’ve been sitting back on that one for now until I’m more comfortable talking shop. I think there is still a distinction between SFR plus ADUs compared to a full-fledged triplex. But the line is getting blurry too. We are in new territory. As lending rules change here, it’ll change what happens in the marketplace and maybe how we think about these things too. I do remember a fourplex that sold a couple years ago that was technically a duplex with two ADUs. The market clearly recognized this property as a full-fledged four-unit property, but I still had big questions here about the appraisal aspect since at the time the GSEs would not allow multiple accessory dwellings on a parcel, so the valuation gets pretty interesting. On one hand, the market was clearly willing to pay big bucks for this place, but we it also didn’t seem like it met requirements for lending….
I was recently on a building tour – two houses on a lot separated by a breezeway. Both houses had complete infrastructure for two families. Question was is this officially :
-Two buildings on a lot?
-A duplex?
-A main house and an ADU?
-Or just on big house for a multigenerational living?
That is a bit frustrating to comp as each variation could be comped differently.
Owner, tax records, appraiser and MLS each have another opinion.
It was a huge lot with a pool.
What to do in a case like that ?
Comp 4 different methods and compare?
Tarah Pahlavan
It can be very tricky to figure out what a property is worth. My first questions: What do permits say it is? What does the building department say it is? If we look at a permit and it says the second unit was built as an ADU, then it’s a no-brainer.
Having a breezeway isn’t enough to consider it attached. Imagine a breezeway that was 1,000 feet in length. I realize this is a crazy example, but I think it’s helpful maybe. Would we really consider these two units separated by 1,000 feet as being the same house? No. And it’s just the same when it’s only five feet away. A good ANSI rule of thumb is this. If you have to step out of the house into something else, then we’re dealing with something else, and we aren’t going to be adding that other structure into the square footage of the primary dwelling. In contrast, if these units were both attached and directly accessible to each other through the interior of the house, and it really felt like one bigger house, then it sounds plausible to include the square footage together (let’s figure out what permits say, of course). In short, I feel pretty confident to say this is not one big multi-generational house.
One frustrating thing in real estate is we sometimes have to deal with conflicting data and conflicting opinions in the real estate space too. Where is truth in the midst of this? We have to ask the right questions and hopefully come up with a reasonable conclusion based on all the information.
How about asking MetroList to add additional options ?
1. Main House SF
2. ADU SF (If more that one ADU, allow for additional options as well)
3. Total SF
Wouldn’t that tell a more accurate version of the total picture ??
Thanks, Steve. I like your thinking.
Locally, we thankfully have number one and two already, but not three. I suspect three might be a stretch, but I do wonder if the MLS space across the country would embrace something like this eventually. I think there needs to be an easy way on the portals to identify the square footage of each respective unit, and that’s a problem right now. Personally, I’m not against a total square footage figure, but I think if we only show that figure without breaking it down, then it’s not an ideal situation. I guess I would rather see something like this:
Primary Sq Ft: 1,800
ADU Sq Ft: 400
I like the idea of trying to emphasize the parts more than the sum because this is way more helpful and constructive to understand what is there. But I wouldn’t put up a fight either if we also had the total.
Hawaii has some interesting rules on ADU or as they call them here, ohana’s. They can attached or detached but cannot have interior access to the main house. Also, if covered breezeway involved it’s added to the sf. Lots of tricky property descriptions to analyze before you even get to comps and value.
Thanks Ryan. Good read on ADU’s
Oh, that is so interesting. I didn’t know about the breezeway especially. Thanks for sharing this, Dan. It’s fascinating how different things can be in various areas. I have to think this gets a little tricky for appraisers in Hawaii when juggling local rules and ANSI for Fannie Mae reports.
This is a great question and thanks for bringing it up. Well, I had a listing that the main house was approx 2200 sq ft 2 bedrooms/1 bth. The ADU was 600 square feet and completely contained with kitchen, bathroom etc. In determining value it doesn’t make sense to compare a 2800 square foot house with 2 bedrooms/1 bath to another 2800 square foot home…when in fact they aren’t equals or comparables. I think pricing the 2200 square foot home and the ADU separately is good possible way to go. Also, MLS needs to indicate ADU and separate out the information for these types of properties from single family…as the ADU gives the opportunity for multi-gen and multi family. But, I am not an appraiser so Ryan I look for the best answer from you.
This is great. I really like the way you approached this, Georgina. And I think agents have so much to offer in terms of value. Thanks for sharing. I like the idea of asking what the primary house would be worth without the ADU and then figuring out how to substantiate an adjustment for the ADU. Let’s look to other sales with ADUs most of all to help tell the story of the market. It’s okay to look at much older sales for research too. We might not use them as comps in a report, but maybe we can milk them for perspective. There isn’t that much data, so we need to do our best and think out of the box. I think the easy route is to look at 2800 sq ft homes and not research the market at all to try to extract and support an ADU adjustment. I think agents would be wise to try to support an ADU adjustment though because that’s what the appraiser is going to have to do when going out to the property (if there is an appraiser involved, of course).
I wish every MLS in the country would get together and agree to adopt ANSI for reporting. It would be better for everyone to have consistent reporting.
It would be very helpful to have an agreed-upon standard. I’m curious to see what FHA and VA decide. I was talking with some MLS people on X, and it sounds like they are thinking through this issue at least.
Agree. At least lets come together and write some rules.
Regarding ANSI; since there is no source for the comparables ceiling heights, no one should be using it. Appraisers using the same comp(s) will look at photos and guess. Some will guess over 7 feet. Some will guess under 7 feet. So F & F will have multiple versions of square footage for these homes. Some appraisers are using the assessor’s square footage. My source is appraisers online(Including Richard Hager). What does this do to the quality of F & F’s portfolio with variable square footage reported for the comps?
Thanks, Jim. The ceiling thing is another layer of data coming soon to appraisal reports. This has not been reported before. I’ll say 99.999% of homes I’ve personally seen have ceilings over 7′, so it’s a pretty safe guess that the comps are going to be over 7 feet in my opinion. It seems like some assessor records are better than others. That’s my observation when talking to appraisers around the country. To your point, junk in…. junk out. We need solid data.
Great post as always Ryan. One thing I have noticed when it comes to creating a discrepancy for the buyer is when they add the bathroom of the ADU into the total bathroom count for the house.
I’ve stepped in many properties with ADUs that were listed as a 3 bed 2 bath, but in truth it was a 3 bed 1 bath with the ADU and a bathroom.
Is there something in MLS outside of square footage to count that extra bathroom without messing up the data?
Hi Marvin, I’ve seen that too. It seems like there are categories for the second unit to include bedroom and bathroom count. It definitely does not help if square footage AND bed/bath count are lumped together with the main structure. If any onlookers have more details, let me know. I just looked up a couple of ADU sales and listings though, and I’m seeing categories for bed and bath for the ADU. I think it depends which report we use though. I use an appraiser view on MLS. The agent full is where I saw greater detail.