"We hired MNBE to build our ADU in Bellevue, and the entire experience was amazing. They handled everything from foundations and framing to kitchen, siding, windows and HVAC."
Zena Heywood
Bellevue, WA · Attached ADU
4.8 STAR RATED40+ CUSTOMER REVIEWS
LICENSED · BONDED& FULLY INSURED
Attached ADU (AADU) builder for Seattle & the Eastside
MNBE designs, permits, and builds attached accessory dwelling units end to end, shared wall construction tied into your existing home across Seattle, Bellevue, Kirkland, Redmond, and the Eastside. Lower cost and a faster build than a from scratch DADU on the same lot.
Shared wall construction
Free written feasibility
Founder supervised
What An AADU Is
An attached unit that shares a wall with your home.
An AADU is an Attached Accessory Dwelling Unit, a self contained second home on the same lot as the primary residence that shares at least one wall with the existing house. It has its own front door, its own kitchen, its own bathroom, and its own legal address. That shared wall is what separates an AADU from a detached DADU. Seattle's zoning code calls it an AADU specifically, while the wider real estate world often says attached ADU, in law suite, or garage apartment, all the same legal classification under Washington state law.
AADUs are permitted up to 1,000 sq ft in Seattle and across the Eastside, can be rented separately, and count toward the two ADU per lot allowance that HB 1337 mandates statewide. Sharing the foundation, roofline, and utility runs with your existing house makes an attached unit lower cost and faster to build than a from scratch DADU, and it works on tight infill lots where a freestanding cottage will not fit. We design, permit, and build the whole thing across Seattle and the Eastside, from Bellevue and Kirkland to Redmond, Sammamish, and Mercer Island.
An attached unit is one of four paths back at the ADU and DADU hub. If your lower level is the obvious candidate, look at a basement ADU conversion; if you want a freestanding rental in the yard, compare a backyard cottage or a full detached DADU. After a free feasibility visit we will tell you which one pencils for your lot, your budget, and your goals.
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4.8 STAR RATED40+ CUSTOMER REVIEWS
LICENSED · BONDED& FULLY INSURED
Seattle AADU Facts · 2026
The numbers behind an attached unit.
An AADU is one half of the two ADU allowance Washington now mandates.
1,000sq ft max
AADU size cap
Room for a real two bedroom unit in Seattle and across the Eastside.
5-8months typical
Build timeline
Signed contract to certificate of occupancy, faster than a comparable DADU.
1
Shared wall
Tied into your existing house, so utility runs stay short and simple.
2
ADUs per lot
An AADU counts as one of the two units HB 1337 allows statewide.
What does an AADU cost?
Priced to your project.
An attached unit usually costs noticeably less than a comparable DADU on the same lot because the foundation, roof, and utility runs are shared with the existing house. The variables that move the bid most are how much existing wall and roof we preserve versus rebuild, whether the existing electrical service supports the second unit load, finish level, and any code mandated upgrades to the existing house the addition triggers, such as egress windows, fire separation, and energy code work. We walk your property first, then put it in writing.
Every MNBE AADU includes
- Tie-in design and structural connection drawings
- Foundation extension and panel upgrade if triggered
- One hour fire rated shared wall with sound attenuation
- Permits, plan check corrections, and inspections
- One fixed price contract, founder supervised
How It Works
What an attached ADU build looks like in practice.
The same five phases on every AADU we deliver, drawn from our finished Bellevue project. No handoffs and no surprises.
01
Tie-in design
We map where the new unit connects, where the shared wall lives, and how existing utilities carry a second unit load, with the firewall and panel detail code requires.
Weeks 1 to 2
02
Foundation extension
The foundation ties into the existing footing or pours as a connected stem wall extension, engineered and inspected, saving time over a freestanding pour.
Weeks 2 to 5
03
Framing & envelope
Two by six walls, continuous exterior insulation, taped and flashed windows. The shared wall gets a one hour fire rated, sound attenuated assembly.
Weeks 5 to 12
04
Systems & interior
Mini split heat pump, heat pump water heater, dedicated subpanel, a real kitchen, and a code compliant bath, so the unit lives as an independent home.
Weeks 12 to 18
05
Handover & warranty
Pathways, lighting, separate address signage, separate meters, and a punch list walkthrough, plus a one year workmanship warranty and full packet.
Weeks 18 to 22
Reviews
What Seattle homeowners say about building with MNBE.
Verified five star reviews from owners who built their ADU, DADU, or renovation with us.
"We hired MNBE for a full renovation of our 100 year old home and the results exceeded expectations, structural upgrades, new insulation, a luxury bathroom, a high end kitchen, a new roof and mini split AC."
Dave Gierok
Sammamish, WA · Whole home renovation
"We hired MNBE for a major exterior renovation: a 900 sq ft custom deck with metal railings, full exterior painting and new siding. From start to finish the experience was exceptional, responsive, meticulous and patient."
Santosh S.
Redmond, WA · Deck & exterior
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4.8 STAR RATED40+ CUSTOMER REVIEWS
LICENSED · BONDED& FULLY INSURED
FAQs · What Clients Ask Us
AADU questions Seattle homeowners ask before they hire.
Practical answers from a builder who runs AADU permits and AADU job sites every week. Call us if yours isn't here. We answer the phone Monday through Saturday.
An AADU is an Attached Accessory Dwelling Unit, a self-contained second home sharing at least one wall with the main house; a DADU is freestanding. Both qualify as ADUs under Washington's HB 1337, run up to 1,000 sq ft in Seattle and the Eastside, and rent separately. The AADU usually costs less but trades privacy and design flexibility.
Three reasons: lot fit, cost, and schedule. A tight lot with critical areas, tree canopy, or restrictive setbacks may not fit a freestanding DADU but supports an AADU. Sharing the foundation wall, roofline, and utility runs cuts the bid, and construction often finishes several weeks faster with no second foundation pour. The tradeoff is privacy across the shared wall.
Seattle caps an AADU at 1,000 sq ft of gross floor area, the same as a DADU, and some lots qualify for a 1,200 sq ft bonus under tree retention rules. Bellevue, Kirkland, Redmond, and Mercer Island also allow 1,000 sq ft. The footprint counts toward lot coverage, and the unit needs its own kitchen, bath, and entry.
Usually not. Most jurisdictions allow shared utilities when the connection is engineered for the combined load: a dedicated subpanel fed from the main panel, which Seattle SDCI permits in most zones, a shared sewer lateral with separate cleanouts, and shared water service. If the existing panel can't carry the second unit, a panel upgrade or separate service is required.
Plan on 5 to 8 months from signed contract to certificate of occupancy for a typical Seattle AADU: roughly 3 to 4 weeks of design, 2 to 4 months of plan check at Seattle DCI, and 2 to 4 months of construction. Eastside cities like Redmond and Kirkland usually run faster on permits, and we hold the written schedule.
We publish every question Seattle homeowners ask us about AADUs, permitting, and construction. Browse the full FAQ library for expert guidance.
View all FAQs
Contact Us · Free Estimate
Tell us about your ADU project.
We'll get back to you within one business day with a clear next step.
Contact information
Thank you for considering MNBE for your ADU or DADU project. We respond within one business day, Monday through Saturday.
(206) 637 0044
info@mnbe-wa.com
2014 Fairview Ave Unit 2308, Seattle, WA 98121