Cities · Snohomish County

Can I build a backyard home in Mill Creek?

Since 2023, Washington law lets most city lots hold up to two more homes. Mill Creek writes its own rules on top. Here’s what the code says, in plain English — and a free report on what may fit on your lot.

Read from the Mill Creek code on October 9, 2026 · confidence medium · a summary, not legal advice. Rules change; the report re-reads them for your parcel.

The short version

What Mill Creek allows.

The headline standards. Open any card for the full wording and the code section.

ADUs per lot2Full wording

2 — 'No more than two accessory dwelling units are allowed per lot.' (MCMC 17.22.130(C)); 'Lots where the only principal dwelling is a single-unit dwelling are allowed two accessory dwelling units.' (MCMC 17.22.150(B)(2))

Max size1,000 sq ftFull wording

1,000 sq ft (min. 400) — 'The gross floor area of accessory dwelling units must not be less than 400 square feet nor more than 1,000 square feet.' GFA = interior habitable space incl. basements/attics, excl. garages (MCMC 17.22.130(B))

Max height35 ft (the NR zone height)Full wording

No ADU-specific height; NR zone maximum 35 ft (MCMC 17.04.100). An accessory building inside a side/rear setback is limited to 400 sq ft and 12 ft; one meeting the principal setbacks takes the zone height (MCMC 17.22.050(C))

Front setback15 ft in NRFull wording

15 ft in NR; 20 ft to a street-facing garage (MCMC 17.04.080(A)); 'No accessory building shall be located in any front setback area.' (MCMC 17.22.050(A))

Side setback5 ft in NRFull wording

5 ft in NR (MCMC 17.04.080(B)); accessory buildings may sit in the side/rear setback but never closer than 5 ft to a side or rear lot line (MCMC 17.22.050(B))

Rear setback20 ft or 20% of lot depth (never under 10 ft); 0 ft at an alleyFull wording

20 ft or 20% of lot depth, whichever is less, never under 10 ft, in NR (MCMC 17.04.080(C)); an ADU may sit on a lot line abutting a public alley (MCMC 17.22.130(D)); 5 ft minimum for accessory buildings in the setback (17.22.050(B))

Lot coverage35% in NRFull wording

35% for single-unit dwellings and duplexes in NR (townhouses 55% buildings / 65% total; multi-unit 40%) (MCMC 17.04.090); conversions of existing structures are exempt (17.22.130(E))

Parking1 space (none near a major transit stop)Full wording

1 space per ADU; 0 within one-half mile (and one-quarter mile) of a major transit stop — Table 17.27.060-1 row 'Accessory dwelling unit | 1 | 0 | 0' (MCMC 17.27.060(A), Ord. 2025-933)

Where a city figure reads stricter than the state floor (RCW 36.70A.681), the floor may control — the report flags it for your parcel.

The rest

Living there, selling it, plans, time and fees.

Each answer is the code’s own wording, with the section it comes from. “Not published yet” means the city hasn’t posted a figure — the report asks the city, not the internet.

Are backyard homes allowed?

Yes — ADUs are a secondary use in NR under MCMC 17.22.130 (MCMC 17.04.040(E)); may be created within existing structures including detached garages 'even if they do not meet setback or lot coverage requirements' (MCMC 17.22.130(E))

Minimum lot size

None for the ADU. NR minimum lot size 7,200 sq ft for all dwelling types (MCMC 17.04.070)

Do you have to live there?

No — no owner-occupancy provision in MCMC 17.22.130 (subsections A–H) or ch. 17.22; RCW 36.70A.681 bars one

Can it be sold on its own?

Not addressed in MCMC 17.22.130 or ch. 17.22; the RCW 36.70A.681 floor requires it — confirm the binding-site-plan / condominium process with the city

Pre-approved plans

None found on the Permits, Land Use Permits & Resources or Community Development pages (2026-10-08) — confirm with city

How long the city takes

Not published — the Building Permits and Permits & Licensing pages give only inspection scheduling (request by 3:30 p.m. for next business day). Ask Development Services for the current estimate (https://millcreekwa.gov/topics/building-permits-0)

Impact fees

'Impact fees assessed on the construction of an accessory dwelling unit must not be greater than 50 percent of the impact fees that would be imposed on the principal unit.' (MCMC 17.48.080, Ord. 2025-933). 2026 Master Fee Schedule (Res. 2025-810): parks $3,308.45 neighborhood + $1,738.67 community per single-family/condo; transportation $3,900 per PM peak-hour trip; school (Everett SD) single-family $9,118, 0–1 bedroom multifamily $0 — no ADU line, apply the 50% cap

Your lot

The code is half the answer. Your parcel is the other half.

Lines and easements

Where the lot lines really are, which setbacks apply on each side, and whether a utility or access easement takes the corner you had in mind.

Critical areas and slope

Steep slopes, wetlands and streams come with buffers that no citywide number shows. The report reads the county’s maps for your parcel.

Sewer, septic, water, trees

Whether sewer reaches the lot or it’s on septic, where the water service is, and which trees a city protects — the things that decide where a unit can actually go.

Free, within a few hours, no obligation.
What Mill Creek allows, what could fit on your lot, and a planning cost range. If something won’t fit, the report says so — and shows what might.

Check my property, free →

Sources

Where this page comes from.

Read from the Mill Creek municipal code and the city’s own pages on October 9, 2026. The same table feeds the HousUp report, so the report and this page never disagree. Cities amend their codes; before you design anything, confirm the current wording with the city.

Nearby

Other places in Snohomish County.

Edmonds 1,200 sq ft Everett Under 1,000 sq ft by definition Lake Stevens 1,000 sq ft gross floor area Lynnwood 1,000 sq ft Marysville 1,000 sq ft or 50% of the residence, whichever is greater, capped at 1,400 sq ft Mountlake Terrace 200-1,000 sq ft excluding garage All cities →

Find out what your property could become.

Type your address. Within a few hours: what Mill Creek allows, what could fit, and a planning cost range. Free, no obligation.

Check my property, free →How it works