Thornberg Construction added an attached mother-in-law apartment to an existing home in Kirkland, WA, completed in 2025. The project gave the homeowners a self-contained addition attached to their existing house, built for multigenerational living where family can stay close while keeping their own space.
An addition like this also falls under a broader category: the accessory dwelling unit, or ADU, the term many municipalities use for a self-contained living space attached to or built alongside a primary home. Whatever it's called, building one means coordinating a design that works for the family with the permitting process that has to sign off on it before construction starts. Thornberg's scope on this project covered that full distance: architectural design, permitting, and construction, along with a remodel of an existing room in the main house to tie the new space into the home's layout.
The main living space has a vaulted ceiling and a teal accent wall that gives the room some presence instead of disappearing into builder-grade white. Wood-look flooring runs throughout, and a built-in cabinet and bench with open cubby storage sits along one wall, the kind of fixed storage a self-contained unit needs since there's no garage or basement nearby to absorb the overflow. A doorway off the main room leads to a utility area with its own sink.
Elsewhere in the unit, a built-in desk and wall cabinetry sit under a wall-mounted mini-split unit, which gives the room its own heating and cooling apart from the main house. Down the hall, a closet is fully fitted with built-in wood-tone shelving, hanging rods, and a drawer tower, the kind of organized storage that makes a smaller footprint livable long-term. The bathroom carries a glass-enclosed shower finished in stone-look tile, alongside a dark vanity topped with a white countertop.
The remodel work on the existing room rounds out the project. Rather than treating the addition as a box tacked onto the house, Thornberg updated part of the original structure so the new apartment reads as part of the home rather than an afterthought bolted to the side of it.
For homeowners in Kirkland considering an attached ADU or mother-in-law apartment of their own, this project is a working example of what that process looks like end to end: design, permitting, construction, and a finished space built for someone to actually live in.
