Can I build a backyard home in Sumner?
Since 2023, Washington law lets most city lots hold up to two more homes. Sumner 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 Sumner allows.
The headline standards. Tap any one for the full wording and the code section.
ADUs per lot2Full wording
2 — 'Two accessory dwelling units shall be allowed per legal building lot' with the one single-family detached dwelling, attached or detached (SMC 18.12.030(A)(1), (A)(5)); none on a lot below the zone's minimum lot size.
Max size1,000 sq ft (garage parking not counted)Full wording
1,000 sq ft maximum floor area, 'excluding any related garage area used for vehicle parking'; the director may allow more for an attached ADU located entirely on a single floor (SMC 18.12.030(A)(3)). A converted accessory structure may be expanded up to 1,000 sq ft (SMC 18.12.030(A)(14)(h)).
Max height24 ft, or the house’s height if that’s lowerFull wording
24 ft for a detached ADU, or the principal dwelling's height if that is under 24 ft (SMC 18.12.030(A)(7)); the LDR zone maximum is 30 ft (SMC 18.12.070(I)).
Front setbackAt or behind the house’s front setbackFull wording
At or behind the primary dwelling's existing front setback (SMC 18.12.030(A)(8)(a)); the LDR zone minimum front yard is 15 ft (LDR-12: 25 ft) (SMC 18.12.070(C)).
Side setback5 ft, or 15 ft combined with two side yards (same as the house)Full wording
Same as principal: 5 ft interior side on a lot with one interior side yard, or 15 ft combined (neither under 5 ft) with two; 12 ft street side (SMC 18.12.030(A)(8)(b); 18.12.070(F)–(H)). A DADU 18 ft tall or less may be 7.5 ft from interior side lot lines (SMC 18.12.030(A)(8)(b)(ii)).
Rear setback30 ft in LDR-6 to LDR-12 (same as the house)Full wording
Same as principal: 30 ft in LDR-6 to LDR-12 (SMC 18.12.070(D)); 15 ft if the DADU is 18 ft tall or less; at the lot line where the rear property line abuts an alley (SMC 18.12.030(A)(8)(b)(i)–(ii)). LDR-4 rear yard is 25 ft (SMC 18.12.075(E)).
Lot coverageLDR-6: 40%Full wording
LDR-6: 40% (45% for single-story homes); LDR-7.2, LDR-8.5 and LDR-12: 35% (40% single-story) (SMC 18.12.070(J)); LDR-4: 40% (SMC 18.12.075(I)).
Parking1 off-street space per ADUFull wording
1 off-street space per ADU; none for ADUs within one-half mile of the Sumner transit station (SMC 18.12.030(A)(4)).
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 — in the LDR districts (LDR-4, LDR-6, LDR-7.2, LDR-8.5, LDR-12) ADUs 'may be located as an attached unit in the same building as the primary dwelling unit or in a detached accessory building' as a permitted accessory use (SMC 18.12.030(A)(5); Ord. 2907, 2025; Ord. 2964, 2026). Other zones' accessory-use sections not read.
Minimum lot size
No ADU on a lot below the zone minimum (SMC 18.12.030(A)(1)). Zone minimums: LDR-6: 6,000 sq ft; LDR-7.2: 7,200 sq ft; LDR-8.5: 8,500 sq ft; LDR-12: 12,000 sq ft (SMC 18.12.070(A)); LDR-4: 4,000 sq ft (SMC 18.12.075(A)).
Do you have to live there?
No — SMC 18.12.030(A), as amended by Ord. 2907 (2025) and Ord. 2964 (2026), contains no owner-occupancy requirement.
Can it be sold on its own?
Not confirmed yet — ask the city. (codified text predates HB 1337)
Pre-approved plans
None found
How long the city takes
Not published
Impact fees
Not found; RCW 36.70A.681 caps ADU impact fees at 50% of the principal unit's
Plans Sumner has already approved.
A pre-approved plan skips the design review: Sumner 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 Sumner 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 Sumner 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.
6 sources, read November 5, 2026
Find out what your property could become.
Type your address. Within a few hours: what Sumner allows, what could fit, and a planning cost range. Free, no obligation.