Building 500 square feet costs about $119,000 as an attached ADU and about $101,000 as a home addition with no kitchen or bathroom at national prices, per the ADU Wizard cost engine (estimates), and about $150,000 as a detached ADU. The addition is cheaper; the ADU can earn rent, which in most of the cities we track pays back that difference within a year or two.
When a family runs out of room, the first instinct is an addition: knock out a wall, add a bedroom and a bathroom, done. The question I always ask back is what that room will be doing in ten years. Because the same square footage, built with a kitchen and its own door, is a second home that can pay for itself.
So here is the comparison I run with homeowners, with the same 500 square feet priced both ways.
Quick Answer: ADU vs Addition
| Home addition | ADU | |
|---|---|---|
| What it is | More rooms in your existing home | A separate home with its own kitchen, bathroom and entrance |
| Cost per sq ft (engine, national, 500 sq ft) | About $202 | About $238 attached, $300 detached |
| Permit path in California | Your zone’s normal rules: setbacks, lot coverage, sometimes design review | Ministerial ADU approval within 60 days of a complete application |
| Typical timeline | Similar to an attached ADU | 5 to 10 months attached, 6 to 12 detached |
| Rent potential | None as a separate unit | Yes, as a separate unit |
| Resale | Adds bedrooms and square footage | Adds a second dwelling an appraiser can value on rent |
| Property tax in California | Only the new construction is reassessed | Same rule |
| Flexibility | It is always part of your house | Family now, rental later, office in between |
Source: ADU Wizard cost engine; California rules from Gov. Code 66317 and 66321 and Rev. and Tax. Code 71; ADU timelines from our type guides
Cost: 500 Square Feet Built Both Ways
| 500 sq ft built as | National range | National per sq ft | California range | California per sq ft |
|---|---|---|---|---|
| Home addition (no kitchen or bath) | $76k to $126k | $202 | $138k to $230k | $368 |
| Attached ADU | $89k to $149k | $238 | $162k to $271k | $433 |
| Detached ADU | $113k to $188k | $300 | $205k to $341k | $546 |
Source: ADU Wizard cost engine, standard finish, all-in. The addition is the attached ADU estimate without its kitchen-and-bath and utility-connection lines. Per sq ft at the midpoint. Estimates, not bids.
The addition is cheaper because it skips the kitchen, the bathroom and the separate utility connection. In our engine’s attached ADU breakdown, those lines are about 15% of the cost. Put a bathroom into the addition and the gap shrinks further. Everything else, the foundation, the framing, the roof and the tie-in to your house, costs the same either way.
Honestly, that gap is smaller than most people expect. Homeowners assume an ADU costs twice what an addition does. On the same footprint attached to the same house, it is closer to a sixth more. A detached ADU costs more again, because it needs its own foundation and a utility trench across the yard, but it also buys the most privacy and the best rent.
Permits: Why an ADU Can Be Easier to Approve
This one surprises people. In California, an ADU often has the easier permit path.
State law requires a city to approve a compliant ADU application ministerially, meaning no hearing and no discretionary design review, and to act within 60 days of a complete application. It caps the side and rear setbacks a city can require for an ADU at four feet, and a city has to allow an ADU of at least 800 square feet even where lot coverage or floor area rules would otherwise block it. An addition gets none of that. It goes through your zone’s normal setbacks, lot coverage and floor area limits, and some cities send larger additions to design review.
That is why I have seen lots where an addition would not fit under the zoning but an ADU of the same size would. The full set of rules is in our California ADU permit guide, and the size floors are on our California ADU size limits page.
Outside California, the ADU advantage depends on your state. Some, like Oregon and Washington, now require cities to allow ADUs by right; others leave it to local zoning.
Value and Resale
An addition adds value the way any bedroom and bathroom do: an appraiser compares your house to houses with the same number of rooms. An ADU adds a second dwelling, and when there are comparable sales or rental income to point to, it can appraise on what it earns.
The honest answer is that it depends on your street, and I went through why in our look at whether an ADU increases property value. The short version: on a block where buyers want rental income or space for family, the ADU wins. On a block of four-bedroom family homes with no ADU comps, a well-done addition can be the safer resale bet.
What About a Guest House?
A guest house is an ADU without a kitchen. It has a bedroom, maybe a bathroom, and its own door in the backyard.
That missing kitchen matters more than it looks. Under California’s definition in Government Code 66313, an ADU has complete independent living facilities, including cooking. A guest house does not, so the state ADU protections (ministerial approval, the setback cap, the 800 square foot floor) do not apply, and your city’s accessory structure rules do. Many cities limit what you can do with one, including plumbing, sleeping use and renting it, so check your local code before you design it.
The trap I see: people build a guest house to save on the kitchen, then want to add one later and find the structure does not meet ADU rules where it sits. If there is any chance it will ever be a home for someone, permit it as an ADU now. Our backyard cottage cost guide prices both versions side by side at 400, 600 and 800 square feet.
The Rent Upside an Addition Can’t Match
Here is the gap no other comparison shows. This table takes the extra cost of building 500 square feet as an attached ADU instead of an addition, at each city’s cost index, and puts it next to what a one-bedroom rents for there.
| City | Extra cost of the ADU over an addition (estimate) | HUD 1BR rent | Rent per year | Months of rent to cover the extra cost |
|---|---|---|---|---|
| Los Angeles | $27,846 | $2,085/mo | $25,020 | 14 |
| San Jose | $39,091 | $2,982/mo | $35,784 | 14 |
| San Diego | $30,880 | $2,459/mo | $29,508 | 13 |
| Portland | $27,846 | $1,677/mo | $20,124 | 17 |
| Seattle | $36,057 | $2,146/mo | $25,752 | 17 |
| Denver | $31,951 | $1,754/mo | $21,048 | 19 |
Source: ADU Wizard cost engine (city index, 500 sq ft attached ADU vs the same addition) and HUD FY2026 one-bedroom Fair Market Rents, metro level, gross rent including tenant-paid utilities.
Read the last column carefully. That is how many months of rent it takes to earn back the ADU premium, before vacancy, taxes and upkeep. After that, the rent is the return on a space an addition would have left empty. Our ADU rental income by city page carries HUD’s rents for 31 cities and the full payback math on a detached unit.
That said, rent only counts if you will actually rent it. If the space will be a bedroom for your own kids for the next 15 years, the rent column is theoretical, and the cheaper addition might be the right call.
Decision Guide: Which One Should You Build?
| Your situation | What I would build | Why |
|---|---|---|
| You need a bigger primary suite and will never rent | Addition | Cheaper, and it connects to the house the way you want |
| Aging parent moving in now, may need care later | Attached ADU | Own kitchen and door, close enough to help |
| Growing family now, rental income later | Attached or detached ADU | It works as family space and becomes a rental without remodeling |
| Your lot cannot fit an addition under zoning | ADU | State ADU rules can allow what the zone will not |
| Want the most value and privacy, have the yard | Detached ADU | Best rent, best resale, no shared walls |
Source: ADU Wizard
If you are leaning ADU, the attached ADU guide is where I would start, since it is the closest thing to an addition that still counts as a second home. For the money side, our ADU financing articles cover how homeowners pay for both, and our latest builder cost data shows how wide real bids run.
Frequently Asked Questions
Is it cheaper to build an ADU or an addition?
An addition, by about 15% on the same footprint. Our cost engine puts 500 square feet at about $101,000 as an addition with no kitchen or bathroom and $119,000 as an attached ADU at national prices. A detached ADU of the same size runs about $150,000.
Does an ADU add more value than an addition?
Often, because it can be valued on rent as well as square footage, but it depends on your neighborhood. Where there are ADU sales or strong rental demand, the ADU usually wins. Where every comparable house is a single-family home with no ADU, an addition can be the safer bet.
Is an ADU easier to permit than an addition?
In California, often yes. A compliant ADU gets ministerial approval within 60 days of a complete application, with side and rear setbacks capped at four feet. An addition goes through the zone’s normal setbacks, lot coverage and sometimes design review.
What is the difference between an ADU and a guest house?
The kitchen. An ADU has complete independent living facilities, including cooking, and gets the state’s ADU protections in California. A guest house has no kitchen, so it falls under your city’s accessory structure rules, which often limit renting it.
Do both raise my property taxes?
In California, both are new construction, and only the new construction is reassessed under Revenue and Taxation Code section 71. The rest of your home keeps its existing assessed value either way.
If I had the lot and any chance of renting it someday, I would build the ADU, even if it starts life as a bedroom wing for the family. The extra cost is small, and it keeps every option open.
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