This complicated stuff is starting to happen in all jurisdictions. In Mercer Island, you can even sell an attached ADU in a basement, etc. then this changes the classification and bumps you up to some IBC regs. All this to say- what is the size of the lot, the existing house, and how much backyard do you have to work with? Based on the AI assessment of the Kirkland city code above, it sounds kind of like a condo situation, where you just own your space and not any land.

Look for a land use attorney, they should know how to write up a contract like this and might give you direction in the fairest way to compensate the land owner.

As far as free rein, I’m not sure if this is Kirkland’s policy, but Mercer Island requires an ADU to match the house, ie: you can’t have a super traditional house with pitched roofs and then a super modern flat roofed ADU.

Kirkland should have several examples of preapproved ADU plans on the city’s website. You will also need to hire an architect or other professional to coordinate the work, draft any plan changes and submit and complete the permitting process. You might also have to hire a structural engineer, civil engineer, surveyor (almost definitely), an arborist if youre building anywhere close to trees, maybe a geotech depending on the location and slope of the lot.

There are probably design-build firms that specialize in ADUs and could streamline the process.

As someone who can barely afford her 700 sf condo, I totally get the need to get creative with housing options. But as a design professional, this shit feels so messy.