Go Far®Why the basement is usually the cheapest legal ADU
An accessory dwelling unit is legally a complete home — kitchen, full bath, sleeping space, its own entrance. Build one from scratch in the backyard and you pay for a foundation, a roof, and four walls before a single cabinet goes in. A basement already has all three, which is why a basement conversion sits at the bottom of the attached-ADU cost range: the budget goes into the kitchen, the bathroom, egress, and sound separation instead of structure. In Seattle these attached units are called AADUs, and thanks to the region’s daylight-basement housing stock, thousands of lots from Shoreline to Renton and Lynnwood to Everett already have one waiting to be finished.
What a basement ADU costs
- Dry, full-height basement with plumbing nearby — the best case: egress and entrance work, a modest kitchen, a full bath tied into the existing stack, sound and fire separation, and finishes. These projects commonly land in the $100,000–$150,000 range.
- Typical full conversion — most basements need more: a new or upgraded entrance, a bedroom egress window and well, panel work, and moisture mitigation before finishes. Expect roughly $150,000–$200,000, still at the low end of what any legal ADU costs in this region.
- Height or water problems to solve — lowering a slab to gain ceiling height, underpinning, or serious drainage work pushes projects toward $250,000+. At that point we price the basement honestly against a detached backyard cottage so you can compare like for like.
For a straight comparison: garage-to-ADU conversions generally run $100,000–$200,000+, and new-build DADUs $250,000–$450,000. If your basement is dry and tall enough, it is usually the value play of the three.
What the code requires
Four requirements decide whether a basement qualifies, and we check all of them in the first site visit. Ceiling height: habitable rooms generally need about 7 feet of finished height, with limited allowances for beams and ducts — re-routing ductwork often rescues a borderline basement, while lowering the slab is effective but expensive. Egress: every bedroom needs a code-compliant egress window or door, which in a basement usually means cutting the foundation and setting a window well. A separate entrance: a self-contained unit needs its own way in — a side or rear door in a daylight basement, or a new stairwell and areaway where grade is against you. Separation: sound and fire separation between the unit and the main house, so both homes feel like homes. Kitchens and baths round out the scope, and homes built before 1978 should carry a contingency for what shows up when walls open.
Permits, and where the rules stand
Every basement ADU we build is permitted — that is what makes it legal to rent and countable at appraisal. Seattle’s rules are among the friendliest of any big city: up to two ADUs per residential lot, no owner-occupancy requirement, and no added parking in most cases (our Seattle ADU & DADU guide has the details). Outside Seattle, Washington’s 2023 ADU law (HB 1337) requires cities inside urban growth areas to allow up to two ADUs per lot and prohibits owner-occupancy mandates — our Lynnwood & Snohomish County rules guide covers the north end. As your general contractor, we prepare the application for the right jurisdiction and meet every inspector on site.
The rent-and-ROI math
A permitted basement ADU is a one-bedroom apartment at Seattle-area rents, created for a fraction of what the same unit costs to build detached. That is why the payback period on a basement conversion is often the shortest in remodeling — and why appraisers and buyers increasingly treat a legal second unit as a hard asset rather than finished-basement bonus space. It is also the least disruptive ADU to build: crews work below while life goes on upstairs, and conversion timelines run months shorter than detached builds. If the goal is aging parents or a returning graduate rather than a tenant, the same unit works as a mother-in-law suite with real separation.
Quick answers
Do I need a permit to turn my basement into an ADU?
Yes — everywhere we work. The permit is what makes the unit legal to rent, insurable, and countable at resale. We prepare the application and handle every inspection.
How much ceiling height does a basement ADU need?
Habitable rooms generally need about 7 feet of finished height, with limited allowances for beams and ducts. Many Northwest basements clear the bar; where one falls short, options run from re-routing ducts to lowering the slab — effective, but the single biggest budget escalator in basement work.
Does a basement ADU need its own entrance?
Yes — a legal ADU is a self-contained home, which means its own entrance and code-compliant bedroom egress. Both are standard scope items in our basement ADU estimates, not surprises.
Can I rent out a basement ADU in Washington?
Yes. Seattle allows up to two ADUs per lot with no owner-occupancy requirement, and HB 1337 extends similar rights across cities in urban growth areas. A permitted unit can be rented long-term from day one.
See what your basement could be
Go Far Construction builds basement ADUs, attached ADUs, and backyard cottages across Snohomish & King County — owners on every job, itemized written pricing, permits handled. Request a free estimate or call (425) 599-3526, and we will tell you honestly whether your basement qualifies and what it would take.