Can I build a backyard home in Des Moines?
Since 2023, Washington law lets most city lots hold up to two more homes. Des Moines 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.
What Des Moines allows.
The headline standards. Tap any one for the full wording and the code section.
ADUs per lot3 — attached, detached or any combinationFull wording
3 — attached, detached or any combination (DMMC 18.55.140(2)). ADUs count toward the lot's unit cap, so a triplex on a 4-unit lot can add only 1 ADU (DMMC 18.55.140 General; 18.57.060(1)).
Max size1,200 sq ft gross floor area maximumFull wording
1,200 sq ft gross floor area maximum; 350 sq ft minimum (DMMC 18.55.140(4)). No exclusions stated.
Max height30 ft — the zone limit for any residential building or structureFull wording
30 ft — the zone limit for any residential building or structure (DMMC 18.55.110); no separate DADU height limit.
Front setback20 ft in RS zonesFull wording
20 ft in RS zones (DMMC 18.55.080); ADUs must meet all underlying-zone setbacks (DMMC 18.55.140 General).
Side setback5 ft each side on lots under 60 ft wideFull wording
5 ft each side on lots under 60 ft wide; 5 ft on one side and 10 ft on the other on lots 60 ft or wider (DMMC 18.55.090).
Rear setback20 ft on lots of 6,400 sq ft or moreFull wording
20 ft on lots of 6,400 sq ft or more; 10 ft on smaller lots (DMMC 18.55.100). A DADU may sit on a lot line that abuts a public alley unless the city routinely plows that alley (DMMC 18.55.140(5)).
Lot coverage35% on lots of 6,400 sq ft or moreFull wording
35% on lots of 6,400 sq ft or more; 2,240 sq ft cap on lots of 4,480–6,399 sq ft; 50% on lots under 4,480 sq ft (DMMC 18.55.120).
Parking0 spacesFull wording
0 spaces — 'Accessory dwelling units: none' (DMMC 18.210.090(15)(i), as amended by Ord. 1821).
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.
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 may be attached, detached or a combination, accessory to a single-family home or a middle housing unit (DMMC 18.55.140(2); 18.57.060(2)). The city lists the zones as RS-15,000, RS-9,600, RS-8,400, RS-7,200 and RS-4,000 (Table 18.52.010A, footnote 52).
Minimum lot size
None for an ADU; zone minimum applies to new lots (DMMC 18.55.050)
Do you have to live there?
No — the replacement DMMC 18.55.140 (Ord. 1820) has no owner-occupancy requirement, and the city's ADU page says owner occupancy is no longer required.
Can it be sold on its own?
Yes — an ADU may be sold or conveyed separately as a condominium unit, independent of the primary unit (DMMC 18.55.140(7)).
Pre-approved plans
None found — the city's ADU page states it does not offer pre-approved ADU templates.
How long the city takes
Not published
Impact fees
All ADUs pay adopted impact fees per the Development and Engineering Services Fee Schedule (DMMC 18.55.140(1)); transportation impact fees on ADUs are limited to the RCW 36.70A.681(1)(a) cap of 50% of the principal unit's (DMMC 12.40.120(3)).
Plans Des Moines has already approved.
A pre-approved plan skips the design review: Des Moines has already checked the drawings, so the permit review is about your lot.
Loading the plans…
Pictures are the official pages’ own. A pre-approved plan is not a permit: the city still reviews your site.
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.
What Des Moines 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.
Where this page comes from.
Read from the Des Moines municipal code and the city’s own pages on November 5, 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.
2 sources, read November 5, 2026
Find out what your property could become.
Type your address. Within a few hours: what Des Moines allows, what could fit, and a planning cost range. Free, no obligation.