Home / Services / ADU & additions / Basement ADU
Bellevue & the Eastside · Basement and interior ADU builder
We convert basements and other parts of the house into self-contained units of up to 500 sq ft, with egress windows, a kitchenette, a bathroom and their own exterior entrance, for one fixed price.
In-house crews · Licensed, bonded & insured in WA · Permits handled for you
Basement ADU at a glance
Typical cost
$60k – $120k
Start to finish
6–10 months
Permit review
Required, we file it
Size
Up to 500 sq ft
Licensed
Bonded & insured in WA
In-house
Crews for the core trades
3D design
See it before we build it
Permits
Filed and managed by us
The short answer
A basement or interior ADU on the Eastside usually costs $60,000 to $120,000 and takes 6 to 10 months from first consult to move-in. It's the lowest-cost way to add a legal unit, because the structure already exists. The unit needs its own exterior entrance, code-compliant egress windows, fire separation from the main house and a certificate of occupancy before you rent it.
Reviewed by John, project lead · Prices from our 2026 Eastside ADU and addition projects · Updated October 2026
Other ADU & addition options
No basement? Compare a garage conversion or a detached ADU in the backyard.
A stand-alone backyard cottage with its own kitchen, bath and entrance, 400 to 1,200 sq ft.
$180k – $320k+
12–18 months
Detached ADUs →
Turn an attached or detached garage into a legal studio or one-bedroom unit, 400 to 700 sq ft.
$90k – $160k
8–14 months
Garage conversions →
Compare every type of ADU and addition we build, side by side, with costs and timelines.
$60k – $350k+
6–18 months
All ADU options →
A new family room, primary suite or kitchen bump-out that adds 300 to 1,500 sq ft to the main house.
$150k – $350k+
10–16 months
Home additions →
Add a full or partial upper floor when your lot can't spare the yard space, after a structural check of the house below.
Quoted per home
Structural check first
Second-story additions →
Lot feasibility, zoning check, 3D design, structural engineering and a permit-ready drawing set.
Design-only available
8–14 weeks
Plans & design →
Basement ADUs on the Eastside
We plan the layout, cut in egress windows, add a separate exterior entrance, build fire separation between the units, run plumbing for a kitchenette and bathroom, add heating and ventilation, and finish it to the standard of the main house. Permit, inspections and certificate of occupancy included.
Eastside basements can take on water. We check for moisture and drainage before design, and fix any problems before finishes go in.
When the designer and the builder are the same company, the drawings are priced as they're drawn. You never pay for an ADU design you can't afford to build, and the crew that frames it helped plan the foundation and utility runs.
✓
Free lot and zoning check before design
✓
3D design and permit-ready drawings
✓
Engineering and city submittal handled
✓
Fixed price before ground breaks
✓
In-house crews, one project lead
Rules and timelines
DADU · 400–1,200 sq ft · 12–18 months
Garage conversion · 400–700 sq ft
Basement ADU · up to 500 sq ft
Addition · 300–1,500 sq ft
Up to 2 ADUs per lot (HB 1337)
No owner-occupancy requirement
Impact fees capped at 50%
Certificate of occupancy before renting
How it works
01
Week 1
We visit, look at access, slope, setbacks and utilities, and check what your city allows on your lot.
02
Weeks 2–10
We design the unit in 3D, size the utilities and complete the structural engineering, then lock in a fixed price.
03
Weeks 10–14
We prepare the full permit set and submit it to your city.
04
Months 3–7
The city reviews the plans, usually 10 to 16 weeks for a DADU. We answer every correction notice for you.
05
Months 7–18
Our crew builds it in 16 to 24 weeks, and we take the final inspection through to a certificate of occupancy.
Typical projects
Detached ADU
A one-bedroom DADU with its own entry and path. Renting at $2,200 a month brings in about $26,400 a year, paying back a $220k build in about 8.3 years.
$180k – $320k+
12–18 months
Garage conversion
Garage door replaced with windows and an entry, new insulation, kitchenette and bath. A $110k conversion renting at $1,800 a month pays back in about 5 years.
$90k – $160k
8–14 months
Interior ADU
A basement turned into a separate unit with a kitchenette, bathroom, egress windows and its own exterior door.
$60k – $120k
6–10 months
Reviews
Real Google reviews from homeowners we've built for across Bellevue and the Eastside.
FAQ
Straight answers on cost, timelines, rules and rental income. Ask us anything else at the free lot check.
How much does a basement ADU cost?
Most Eastside basement and interior ADUs cost $60,000 to $120,000. Egress windows, how far plumbing has to run and any moisture fixes move the number most.
How long does a basement ADU take?
Usually 6 to 10 months from first consult to move-in, including design, permit review and construction.
What did Washington's HB 1337 change for ADUs?
HB 1337, passed in 2023, requires Eastside cities to allow at least two ADUs per lot, drop owner-occupancy rules, cap ADU impact fees at 50% of a new house's, and allow detached ADUs of at least 1,000 sq ft. Cities had to update their codes by 2025, so details still vary a little by city. We check yours at the lot check.
Can I rent out my ADU?
Yes. With no owner-occupancy rule, you can rent the ADU or the main house. You need a certificate of occupancy first, which we get at final inspection. Eastside rental vacancy runs under 5%, and a DADU renting at $2,200 a month pays back a $220k build in about 8.3 years.
Does an ADU add value to my home?
Usually, yes. A permitted, well-built ADU on the Eastside typically adds $150,000 to $280,000 in appraised value, on top of the rent it can bring in.
Should I build an ADU or a home addition?
Build an ADU if you want a separate unit with its own kitchen and entrance for rent, family or guests. Build an addition if you want more room in the main house, like a bigger kitchen, a family room or a primary suite. We'll sketch both on your lot at the free consult.
What if my lot is sloped?
Lots steeper than about 15–20% need a geotechnical report before the design is final. Plan on $3,000–$6,000 and 4–6 extra weeks. We order it and design the foundation around what it finds.
Start your basement ADU
Tell us about your property. We'll visit, check zoning, slope and utilities, talk through what you want to build, then send you a fixed price.
Renovision Design & Build LLC
1400 112th Ave NE, Bellevue, WA 98004
(425) 276-6501 · Mon–Fri 8–5
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