An in-law suite inside the house you already own.
A basement conversion or attached addition turns unused square footage into a separate, legal unit — a mother-in-law suite, an apartment for an adult child, or a long-term rental — usually for less than building a detached cottage. The catch is four things most quotes don't check: ceiling height, egress, moisture, and a separate entrance.
Is this you?
You have a basement that’s storage, or a footprint to bump out, and you want a separate space without a new building in the backyard. Often it’s about keeping a parent close: “keep Mom nearby but not in my house.” An attached ADU gives that separation under one roof — and tends to cost less than a DADU because the shell is already there.
This format fits if you want to:
- House an aging parent or an adult child in a private, separate unit attached to the home.
- Add rental income using a basement or addition instead of a detached build.
- Keep the budget below a full DADU when you have usable existing square footage.
What’s included — and what’s not
| Included | Not included |
|---|---|
| Separate entrance, kitchen, bath, egress windows | The land — you already own it |
| Waterproofing + moisture control (the basement essential) | Major utility upgrades beyond a standard connection |
| Insulation, drywall, HVAC to current energy code | Lowering the slab if ceiling height fails (priced separately) |
| Electrical, plumbing separation, permit package | Furniture, appliances above standard, extra landscaping |
| Inspections + standard utility connection | Sound insulation upgrades beyond the standard set (optional add) |
Published price range
All-in covers the full conversion or addition minus the land. A basement conversion can start lower than a detached build because the structure exists; a new attached addition with its own foundation lands higher. Standard finish, 2025–26 King County ranges.
| Size | Vertical build | All-in (standard) |
|---|---|---|
| 500 sf basement studio | ~$110K–$165K | $150K–$230K |
| 650 sf 1BR | ~$140K–$215K | $195K–$300K |
| 800 sf 2BR / attached addition | ~$175K–$265K | $240K–$370K |
This is a planning range from real project data, not a bid. Your number depends on site conditions, finish level, and design. We confirm an exact, fixed-scope price on a real quote.
How it works
- Feasibility — the four gates: ceiling height, legal egress, moisture, and a separate entrance.
- Design + permit package — sized to the existing space and your use.
- Permit review — typically a few months through SDCI or your city / DPER.
- Build — waterproofing, egress, kitchen, bath, separation of utilities, finishes.
- Inspections + utility connection, then a legal, rentable attached unit.
City notes
Basement and attached ADUs are under-covered in most builder marketing, even though they’re often the cheapest legal unit. Under HB 1337, an attached ADU counts toward the two-ADU-per-lot allowance across King County — so you can pair a basement AADU now with a backyard DADU later. In Seattle, permitting runs through SDCI; in unincorporated King County, through DPER, where on-septic lots need a capacity check before adding a unit. Owner-occupancy and parking requirements are gone, which makes renting an in-law suite simpler than it used to be.
An AADU is an attached accessory dwelling unit — built inside or onto the main house, like a basement conversion or an attached addition with its own entrance. A DADU is detached — a separate building in the backyard. AADUs usually cost less because they reuse existing structure; DADUs offer more privacy and independence.
