Should an ADU be included in the square footage?

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. 

UPCOMING SPEAKING GIGS:

8/25/26 Elk Grove MLS Meeting 8:30am
9/1/26 ROG Talks
9/15/26 Culbertson & Gray
10/2/26 PCAR Rocklin
10/21/26 Coldwell Banker Sierra Oaks / EDH

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.  

Thanks for being here. 

Question: Do you think an ADU should be separate or combined with the square footage of the main house? 

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