How much does it cost to add a mother-in-law suite in Washington?

It depends on the route. In the Seattle–Snohomish area: converting a basement runs roughly $70–$150 per square foot; a garage conversion runs about $30,000–$60,000 as a room, $60,000–$100,000 with a bathroom, or $100,000–$200,000 as a full ADU; an attached in-law addition typically runs $150,000–$275,000; and a detached backyard cottage (DADU) commonly lands in the $250,000–$450,000 range.

Is a mother-in-law suite the same as an ADU?

Not always. A suite that shares the home's kitchen is simply part of the house and permits like any remodel or addition. Once the space has its own full kitchen, it is an accessory dwelling unit — a legal second dwelling with its own code requirements, and the flexibility to become a rental later. Washington's statewide ADU law now guarantees homeowners in urban growth areas the right to add ADUs.

Can a mother-in-law suite earn rental income later?

If it is built as a legal ADU with its own kitchen and entrance, yes — Washington's HB 1337 removed owner-occupancy requirements in urban growth areas, so a suite built for parents today can generally become a rental later. A suite without a kitchen adds living space and value but cannot be rented as a separate dwelling.

What is the best mother-in-law suite option for aging parents?

Single-level and step-free matters more than which structure it is. A ground-floor suite — converted, added on, or a one-story backyard cottage — with a curbless shower, wide doorways, and good lighting serves aging parents best. Detached cottages offer the most privacy; attached suites keep family closest for caregiving.

Mother-in-Law Suite in Washington: Options & Costs (2026 Guide)

First, what “mother-in-law suite” actually means

The term covers everything from a bedroom with its own bath to a fully independent apartment. The dividing line that matters for permits and cost is the kitchen: a suite that shares the home’s kitchen is simply part of the house, while a space with its own full kitchen becomes an accessory dwelling unit (ADU) — a legal second dwelling with its own code requirements, and its own long-term flexibility as a rental. Both are worth considering, and Washington’s statewide ADU law (HB 1337) now guarantees homeowners in urban growth areas the right to add ADUs, with no owner-occupancy requirement and friendlier height and parking rules than a few years ago.

Option 1: Convert space you already have

The most budget-friendly route is turning existing square footage into a suite. A basement converts at roughly $70–$150 per square foot depending on egress, moisture work, and ceiling height (our basement remodel guide covers what drives that range) — an 800-square-foot basement suite with a bedroom, bath, and sitting room commonly lands in the $60,000–$120,000 band, more with a kitchen. A garage converts to a conditioned room for about $30,000–$60,000, reaches $60,000–$100,000 with a full bathroom, and becomes a complete legal ADU with kitchen at $100,000–$200,000 — the full math is in our garage conversion guide.

Option 2: Build an attached in-law addition

When there is no basement to claim and the garage needs to stay a garage, an attached addition adds purpose-built space. New living space in the Seattle–Snohomish area prices at roughly $300–$550 per square foot, so a 400–600-square-foot suite — bedroom, full bath, sitting area, kitchenette — typically runs $150,000–$275,000 depending on finish level and site. The attached route keeps parents literally next door, lets the suite share the home’s systems, and is the natural choice when daily connection is the point. Our home addition cost guide and primary suite guide break down where those dollars go.

Option 3: Build a backyard cottage (DADU)

For maximum independence, a detached ADU — a backyard cottage — is its own small home: complete kitchen, laundry, private entrance, and a door that closes. Complete DADU projects in the Seattle–Snohomish area commonly land in the $250,000–$450,000 range depending on size, site, and finish, with compact projects below that band — see our DADU cost guide. It is the most expensive option and also the only one that adds an entire dwelling to your property — housing for parents now, a rental or guest house later. Families weighing it against senior-living costs often find the cottage pencils out surprisingly fast, and our ADU financing guide covers how households actually fund them.

Design it for the person, not just the plan

A suite for aging parents earns its keep through details: single-level and step-free, a curbless shower with blocking for grab bars, 36-inch doorways, bright glare-free lighting, and lever hardware — none of which needs to look clinical. Our aging-in-place guide covers these choices in depth. They cost very little during construction and everything to retrofit later, so we design them in from day one.

How families actually choose

The pattern we see across Lynnwood, Edmonds, Bothell, Kirkland, and the Eastside: convert when the house has a good basement or a spare garage bay and the budget leads the decision; add on when daily closeness matters most — shared meals, caregiving, grandkids; build a cottage when everyone values independence and the long-term rental flexibility justifies the bigger number. Go Far Construction builds all four versions across Snohomish and King County, and a free estimate is often what settles the question — seeing the real numbers for your lot and your house side by side. Tell us who the suite is for, and we’ll help you find the version that fits.

Mother-in-Law Suites in Washington: Every Option, With Real Costs