Can I build a backyard home on my lot? A checklist

Most people who ask us whether a second home fits in their yard already suspect the answer. They have stood at the back fence and pictured it. What they don’t have is a way to check their suspicion without paying a designer first.

This is that check. It’s the same list our report runs against public records, written so you can run the first pass yourself with a tax-parcel map and a walk around the lot. None of these eleven is a verdict on its own. Several of them are reasons to stop guessing and get the report.

First, is the state rule on your side?

Since 2023, Washington law (RCW 36.70A.681) requires cities and counties to allow at least two accessory dwelling units on lots inside an urban growth area, in zones that allow single-family homes. It sets a floor your city cannot go below: a size cap no lower than 1,000 square feet, a height limit no lower than 24 feet, setbacks and lot coverage no stricter than for the main house, no owner-occupancy requirement, and no parking requirement within half a mile of a major transit stop. We walk through the whole statute in What HB 1337 actually changed.

Two carve-outs matter before anything else on this list. The requirements do not apply to lots with critical areas or their buffers (wetlands, streams, steep slopes, landslide areas), and they do not apply outside the urban growth area. Item 1 and item 7 below are those two carve-outs.

The eleven things

1. You are inside the urban growth area, in a residential zone

Look up your parcel on your county’s assessor map and note the zoning. In Everett the residential zones are labeled NR, UR and MU; in unincorporated Snohomish County they are R-9,600, R-8,400 and similar. If your lot is outside the urban growth area, HB 1337’s requirements do not apply; a 2026 bill (HB 1345) lets counties optionally allow one detached unit per lot outside the UGA, but “optionally” is the operative word.

2. There is room left after setbacks and lot coverage

Buildable area is setbacks and lot coverage together. Take your lot size, subtract the house and any structures, and compare the result to your city’s coverage limit. A 7,200-square-foot lot with a 2,000-square-foot house at a 35 percent base coverage has very little room. The same lot in unincorporated Snohomish County, where coverage goes to 55 percent when an ADU is added, has meaningfully more. Then rear and side setbacks (5 feet each in the common Snohomish County zones) carve what’s left into a shape, and the shape has to fit a building.

3. A truck can reach the back of the lot

This is the single most common surprise. A person, a small excavator and a concrete pour all have to get to the build site. Alley access is gold. A side yard wide enough for equipment is fine. A shared driveway is a negotiation with your neighbor. No rear access at all often means an easement conversation or a much more expensive build.

4. The rear yard is reasonably flat

Slope drives foundation cost, and a noticeable slope can trigger a geotechnical report, typically $3,000 to $7,000 when one is required. A retaining wall that is doing real work is a clue. Sloped lots on the Eastside are a known cost driver across every builder’s published range.

5. The trees are not in the only place a building could go

Municipal tree protection and removal rules can make an otherwise buildable rear yard unusable. A large protected tree with no rear access is the classic dead end. Note the diameter and species of anything big in the rear third of the lot; the report checks it against your city’s retention rules.

6. Sewer or septic can take a second home

A backyard home needs water, sewer or septic, and power. Unincorporated Snohomish County requires documentation that the existing or proposed sewage disposal and potable water can handle the added demand, with Health Department review for septic. State law preserves a city’s right to prohibit ADUs on unsewered lots. If you are on septic, the question is whether there is room for a replacement drainfield; if you are on sewer, the question is where the lateral runs and what a connection costs (roughly $5,000 to $15,000 in Seattle, per Seattle ADU Builders’ published permitting guide).

7. No wetlands, streams, steep slopes or landslide areas

Critical areas and their buffers are the hardest stop. HB 1337’s requirements explicitly do not apply to them. Standing water, cattails, alder thickets, or a neighbor’s property that drains onto yours are the field signs; the county’s critical-areas map is the desk check. A property in a critical area is not automatically dead, but it is a different, slower and more expensive project, and one a preliminary report cannot resolve.

8. No recorded covenant or HOA rule against it

State law overrides city zoning. It does not override a private covenant. If your plat has CC&Rs, or you pay HOA dues, read the section on accessory structures before you do anything else.

9. You know what’s under the ground and over it

Recorded easements (utility, access, drainage) can cut a buildable yard in half on paper. Overhead power lines set clearance requirements. Both are on your title report and your plat; neither is visible from the fence.

10. Nothing was built without a permit

Anything that starts with “the previous owner built that without a permit” needs to be resolved before a new permit is issued on the same lot. It’s usually solvable. It is never free.

11. You know the four rules your city sets above the state floor

The state sets the floor; your city sets the actual numbers. The four that decide the most are the maximum size (Seattle: 1,000 square feet, or 1,200 with three or more bedrooms; Everett: a unit under 1,000 square feet is an ADU, and a larger one is simply another dwelling; unincorporated Snohomish County: 1,200 square feet, excluding garage, porches and unheated space), the height limit (24 feet minimum by state law; 32 feet in Seattle’s neighborhood residential zones; 30 feet in Snohomish County’s R-9,600 and R-8,400), the setbacks (Seattle: 5 feet from lot lines, 0 at an alley; Everett: the same as the principal house; Snohomish County: front 10, side 5, rear 5), and parking (none in Seattle; none in Everett for units under 1,200 square feet; none in Snohomish County’s urban zones). Every one of these jurisdictions changed its code in the last eighteen months, and several more are changing now, so treat any number you read online, including these, as a thing to verify on the date you build.

What to do with your answers

If you got through the list without a hard stop, the report is the next step, and it’s free. It runs these same checks against the assessor’s records, the zoning code, FEMA and the county’s critical-areas layers, and it tells you which size band may fit and what a project like it has cost recently nearby. If it can’t tell, it says what would settle it.

If you hit a hard stop, the report is still worth having, because “no” with the reasons is what lets you stop wondering, and because the addition or the garage conversion that does work on your lot is often the answer the report gives instead.