A backyard cottage built as a detached ADU costs about $120,000 for 400 square feet to $240,000 for 800 square feet at national prices ($218,000 to $437,000 in California), per the ADU Wizard cost engine (estimate). Without a kitchen it is a guest house, which costs less but usually cannot be rented as its own home.
Last verified: October 19, 2026
"Backyard cottage" is what most people picture when they say ADU: a small, separate house at the back of the lot, with a porch and its own front door. It is a lovely word, and it hides the one decision that sets your cost, your permit and what you can do with the building later. Does it have a kitchen?
With a kitchen, bathroom and entrance, a backyard cottage is a detached ADU, a real second home. Without a kitchen, it is a guest house, which your city treats as an accessory structure like a big shed or a studio. Here is what each costs, how each gets permitted, and how I would design one.
Backyard Cottage Cost
Both versions at the three sizes I build most, from our cost engine at standard finish. The guest house figure removes the kitchen, bathroom and separate utility service from the detached ADU estimate.
| Size | Cottage as a detached ADU, national | Detached ADU, California | Guest house (no kitchen or bath), national | Guest house, California |
|---|---|---|---|---|
| 400 sq ft | $90k to $150k | $164k to $273k | $76k to $126k | $138k to $229k |
| 600 sq ft | $135k to $225k | $246k to $410k | $113k to $189k | $206k to $344k |
| 800 sq ft | $180k to $300k | $328k to $546k | $151k to $252k | $275k to $459k |
Source: ADU Wizard cost engine (standard finish, all-in). The guest house column is the detached estimate without its kitchen-and-bath and utility-connection lines; add a bathroom back and it moves toward the ADU number. Estimates, not bids.
The gap between the two columns is smaller than people expect, because the kitchen and bathroom are only part of the build. The foundation, framing, roof and windows cost the same either way. That is why I usually tell people that if they are spending the money on the building, they should spend the rest on the kitchen and make it an ADU.
Prefab and Kit Cottages
A lot of the cottages in the search results are factory-built. In California, our prefab data puts an installed prefab unit at $145k to $230k for 400 square feet and $250k to $420k for 800, including foundation, delivery and hookups. Our prefab ADU guide splits that into the factory and site costs, and the ADU kits guide covers shell kits, which are cheaper up front but leave most of the work, and the permit, to you.
Backyard Cottage vs Guest House: Which One Are You Building?
| Cottage permitted as a detached ADU | Guest house (no kitchen) | |
|---|---|---|
| Kitchen | Yes, a full kitchen | No |
| Legal category | A dwelling unit (in California, an ADU under Gov. Code 66313) | An accessory structure under local zoning |
| Permit path in California | Ministerial ADU approval within 60 days of a complete application (Gov. Code 66317) | Your city's accessory structure rules; the state ADU protections (ministerial approval, the setback cap, the 800 sq ft floor) do not apply |
| Renting it out | Yes, as a separate home (cities may set a minimum term) | Usually not as a separate dwelling |
| Cost, 600 sq ft, national | $135k to $225k | $113k to $189k |
| Resale | Counts as a second dwelling an appraiser can value | Counts as an outbuilding with living space |
Sources: California Government Code 66313 and 66317 as covered in our ADU vs home addition guide; costs from the ADU Wizard cost engine.
This is the part nobody warns you about. People build the guest house to save on the kitchen, then add a hot plate, and now they own an unpermitted unit. If there is any chance the cottage becomes a home for a parent, a grown kid or a tenant, permit it as an ADU now. Our mother-in-law suite guide walks through the same choice for a family unit, and the ADU vs addition comparison covers the guest house trap in more detail.
Permits for a Backyard Cottage
As an ADU, a backyard cottage gets the strongest protections state law offers in places like California: a city has to decide a complete application within 60 days (Gov. Code 66317) and must allow a detached unit of at least 800 square feet. Our California ADU size limits page covers the exact numbers, and the ADU permit cost guide covers the fees.
Outside California, the rules come from your state and city. Washington's HB 1337 requires cities to allow ADUs, and Seattle runs a pre-approved backyard cottage plan program; our Washington ADU law guide covers both. Our list of states that allow ADUs has every statewide law with its citation.
Designing a Backyard Cottage That Works
- Face it toward the yard, not the fence. A porch and windows on the garden side make 400 square feet feel twice the size.
- One bedroom is the sweet spot. A studio rents, but a one-bedroom rents to more people and works for a parent.
- Put the path in the plan. A short, flat, lit walk from the street or the house gets used every day.
- Vaulted ceilings, not more square feet. Volume is cheaper than floor area and the room feels bigger.
- Plan the utilities first. Sewer and power runs across the yard are where cottages go over budget.
Honestly, the cottages I like best are the simplest: a rectangle, one roof line, good windows. Every jog in the footprint adds framing and roofing cost without adding much living space. Our 1-bedroom ADU floor plans are drawn to scale if you want to test a layout before paying a designer.
Frequently Asked Questions
How much does a backyard cottage cost?
Built as a detached ADU with a kitchen and bathroom, about $120,000 for 400 square feet to $240,000 for 800 square feet at national prices, and about $218,000 to $437,000 in California, per our cost engine (estimate). A guest cottage with no kitchen or bathroom runs about $101,000 to $202,000 nationally.
What is a backyard cottage called?
Legally, usually a detached accessory dwelling unit, or detached ADU. Seattle calls them DADUs or backyard cottages, and you will also hear granny flat, casita and guest house. If it has no kitchen, most cities treat it as an accessory structure, not a dwelling unit.
Do you need a permit for a backyard cottage?
Yes, if anyone will live or sleep in it, and almost always if it has plumbing or more than a small footprint. In California, a cottage with a kitchen is permitted as an ADU, which gets ministerial approval within 60 days of a complete application (Gov. Code 66317). A guest cottage without a kitchen goes through your city's accessory structure rules instead.
What is the difference between a backyard cottage and a guest house?
The kitchen. A cottage with its own kitchen, bathroom and entrance is a dwelling unit, an ADU, and can be rented as one. A guest house without a kitchen is an accessory structure: cheaper to build, but you lose the state ADU protections and usually the right to rent it as a separate home.
How long does it take to build a backyard cottage?
Plan on 6 to 12 months from first design meeting to move-in for a detached ADU. A prefab cottage overlaps the factory build with your foundation, but design, permits and site work still come first.
If I were building a backyard cottage tomorrow, I would build it as a one-bedroom detached ADU, permitted as an ADU, with a full kitchen. It costs a bit more than a guest house and does a lot more. Start with the detached ADU guide for the cost by size and state.