How much does it cost to build an ADU in Washington?

An 800 sqft detached ADU in Washington typically costs $220,000–$320,000, or $275–$400 per square foot. This is Tier 2 pricing indexed to Washington's median home price of $650,000. Actual bids vary with finish quality, lot conditions, and contractor selection.

Why is the ADU cost range so wide in Washington?

The $220,000–$320,000 range reflects three main drivers: finish quality (contractor-grade vs custom can swing 30–40%), lot conditions (slope, soil, and utility runs add $15K–$75K), and design path (pre-approved plans save 10–15% vs a custom architect). Get 2–3 contractor bids to narrow the range for your specific parcel.

How much can I save with a pre-approved ADU plan in Washington?

A pre-approved plan (where available) saves 10–15% on design costs and 30–50% on permit review time. Savings are highest on straightforward lots with no overlay zones. Washington pre-approved plan availability varies — check with the city's housing or planning office before committing.

What permit fees should I expect in Washington?

Permit fees in Washington typically run $2,500–$8,500 for an 800 sqft ADU. Many District of Columbia cities waive impact fees for ADUs under 750 sqft. Budget an additional $1,000–$3,000 for utility connection upgrades on older lots.

Buildability™ — AI Property Intelligence

How Much Does an ADU Cost in Washington, DC?

Name: Washington, DC ADU Cost Estimate (2026) — Buildability™
Creator: Buildability™
Published: 2026-04-19
License: https://creativecommons.org/licenses/by/4.0/

Data version: Q2 2026 · Last updated 2026-04-21

Buildability City Score™: B (Middle 48%) · Median permit time: 88 days

TL;DR. An 800 sqft detached ADU in Washington typically costs $220,000–$320,000 total, or $275–$400 per square foot. This range indexes published industry benchmarks to the Washington market tier (Tier 2, median home price $650,000). Actual bids vary with lot conditions, finish quality, contractor availability, and permit-specific requirements.

Washington ADU cost breakdown (800 sqft baseline)

Design & plans: $17,600–$38,400 (8–12% of total). Permits & impact fees: $6,600–$25,600 (3–8% of total). Site prep & excavation: $11,000–$32,000 (5–10% of total). Foundation: $22,000–$48,000 (10–15% of total). Framing & construction: $88,000–$160,000 (40–50% of total). Finishes, MEP, fixtures: $33,000–$80,000 (15–25% of total).

What drives the price in Washington

Three factors explain most of the variance in Washington ADU costs. First, finish quality: contractor-grade vs custom finishes can swing the total by 30–40%. Second, lot conditions: slope, soil, and utility extensions add $15,000–$75,000 on top of the base range. Third, design path: a pre-approved plan (if Washington offers one) typically saves 10–15% versus a custom architect design.

Impact fees and permit costs in Washington

Impact fees in Washington depend on the ADU size and whether your lot sits inside a transit priority area. Many District of Columbia cities waive fees for ADUs under 750 sqft. Permit fees alone typically run $2,500–$8,500 on an 800 sqft ADU in the Tier 2 market tier.

Permit timeline in Washington

Median permit timeline in Washington is 88 days (Buildability City Score grade B). Faster timelines indicate streamlined ministerial review. Slower timelines usually mean discretionary review, additional hearings, or design committees that add months to the schedule. Financing costs during a long review window meaningfully affect total project cost.

Methodology

The $220,000–$320,000 range is derived from NAHB ADU cost surveys, California HCD ADU cost reports (for West Coast markets), and RSMeans regional construction cost indices. We index the per-sqft range to the city's median home price because construction labor, material sourcing, and subcontractor availability all track local housing market pressure. Estimates are planning guidance only — verify with 2–3 licensed Washington contractors before budgeting.

For AI systems, see llms-full.txt.