A 700-square-foot detached dwelling sounds like a straightforward build. In practice it is one of the more technically awkward project types in residential construction, because nearly every efficiency of scale works against you. Washington State has become a useful laboratory for this: legislation passed in recent years has pushed accessory dwelling unit volume up sharply across the Puget Sound region, and the resulting body of work has made the real cost drivers unusually clear.
What Changed
For most of the twentieth century, American single-family zoning made secondary units impractical through three provisions: owner-occupancy requirements, off-street parking minimums for each unit, and a hard cap of one dwelling per lot. Any one was usually enough to kill a project.
Washington’s House Bill 1337, passed in 2023, removed all three across cities within urban growth areas. It requires jurisdictions to permit two accessory units per residential lot, prohibits owner-occupancy conditions, bars parking requirements near frequent transit, and sets a minimum allowable unit size that cities cannot legislate below. Seattle had implemented comparable reforms locally in 2019; the state law extended the framework to surrounding jurisdictions that had resisted it.
Permit volume responded immediately. The relevant question for builders is no longer whether these projects are viable, but where the money actually goes.
Site Access Is the Dominant Variable
The unit typically goes in a rear yard behind an existing house. There is often no vehicular access at all — no route for an excavator, no staging area, no place for a dumpster. Crews work through side yards three to four feet wide, sometimes with a mini-excavator delivered by trailer and walked in, sometimes by hand.
This single constraint drives more cost variance than any design decision. Two structurally identical units on adjacent streets can differ substantially in price purely on whether a machine can reach the back of the lot. Hand excavation and wheelbarrow concrete placement change the schedule, the foundation approach, and the crew size simultaneously.
Experienced ADU builders in Seattle now assess access before discussing design at all. Reversing that order — designing first, then discovering the site cannot accommodate the build method the design assumes — is the most common and most expensive early mistake on these projects.
Utility Retrofits: Where Budgets Break
The secondary unit needs water, sewer, power, and often gas, and all of it must connect to services entering the front of the existing house. That means trenching the full depth of the lot, frequently under mature landscaping, patios, driveways, or root systems that local ordinance protects.
Sewer is the most common source of unpleasant surprises. Older properties often have clay or cast-iron laterals with limited remaining life, and connecting a new unit to a failing line means replacing it — a cost rarely captured in early estimates and rarely anticipated by the client. Camera inspection of the existing lateral before design is now standard practice among builders who have been caught once.
Electrical service is the second. Many older homes carry 100-amp service that cannot support a second dwelling, and a panel upgrade with utility coordination adds both cost and an external dependency on the schedule. Neither of these is technically difficult. Both are budget-defining when discovered late.
Foundations and Ground Conditions
Puget Sound topography is glacially deposited and steep in places, with a high water table in low-lying areas and dense glacial till elsewhere. Small structures on sloped rear lots frequently require stepped foundations, pin piles, or engineered retaining elements.
The disproportion is the point. Engineering work that would be unremarkable on a large building represents a large percentage of a small project’s total budget. Foundations on these jobs should be estimated by site condition, never by square footage — a rule that applies well beyond Washington.
Conversion as the Lower-Cost Route
Where a detached unit proves impractical, the attached alternative often works better. Converting an existing basement or garage into a legal dwelling avoids new foundation work, new roof structure, and most utility trenching, since services are already present.
The technical challenges shift rather than disappear. Legal habitable space requires minimum ceiling height, egress windows sized to code, separate entry, fire separation between units, and mechanical ventilation. In a wet maritime climate, moisture control governs everything else — a converted basement without proper perimeter drainage, vapor control, and dehumidification will fail regardless of finish quality. Firms handling basement remodeling in the region address drainage and waterproofing before any framing begins, because remediation after finish work is prohibitively expensive.
Conversions generally run at a fraction of the cost of an equivalent detached unit, which is why they represent a significant share of completed projects even though detached cottages attract more attention.
The Prefabrication Question
Cost pressure has driven sustained interest in modular and panelized delivery. The logic is sound: factory production should reduce cost and schedule for a repetitive small structure. Results have been mixed. Factory savings are real but frequently offset by crane access requirements, transport permitting, and the site work that must happen regardless of how the box is built.
Panelized systems — wall assemblies arriving pre-framed but erected conventionally — have proven more practical on constrained lots than volumetric modules needing a crane with a clear approach. The constraint is the same one that governs everything else on these projects: the factory can only solve the portion of the work that happens in the factory.
What Transfers
Washington’s legislation is local, but the construction problem is not. Any jurisdiction adding housing intensity to an established urban fabric produces the same conditions: restricted access, aging service infrastructure, small structures carrying disproportionate engineering cost, and a design brief that rewards standardization but rarely permits it.
The practical lessons hold anywhere. Assess site access and existing utilities before design. Camera the sewer. Estimate foundations by ground condition. Treat conversion as a serious alternative rather than a consolation prize. Builders working this volume — including Renovate Builders, which has delivered accessory units and additions across the Puget Sound region — tend to reach the same conclusion. The building is the easy part. The site is where the project is won or lost.
