How Big Can an ADU Be in Boise?

How big can an ADU be in Boise?

Boise's residential code caps accessory dwelling units at 900 square feet of finished living area (raised from 700 in December 2023). The city's six pre-approved plans top out at the 695 sq ft Kestrel — the largest design that fits inside the cap. Anything larger requires a different review path under the residential code and rarely receives approval as a standalone accessory dwelling.

What counts toward the 900 sq ft cap?

• Conditioned living space (heated, cooled, finished interior)
• Bedrooms, bathrooms, kitchens, living rooms, dining areas
• Hallways and interior circulation
• Finished stair area on two-story plans

What does NOT count toward the cap?

• Attached garage space (the Sandpiper and Osprey garage square footage doesn't reduce dwelling cap)
• Unfinished basements (rare on Boise ADUs, but allowed)
• Covered porches and patios that aren't fully enclosed
• Detached storage structures (sheds, separate workshops)

Why the 900 sq ft cap exists

The cap is a policy choice to keep accessory dwellings genuinely accessory — secondary to the principal residence rather than a competing primary dwelling. Cities that lift or remove the cap entirely (Portland, Los Angeles) end up with effectively two primary residences on one lot, which raises separate land-use questions around parking, density, and infrastructure load. Boise's 900 sq ft cap (raised from 700 in the 2023 Modern Zoning Code) keeps ADUs in the 'small' category and makes the program politically durable.

Pre-approved plans by size

What if I want a larger ADU?

Custom designs over 900 sq ft enter a different review path. They typically require either a variance request (rarely granted), a different parcel configuration (lot split into two parcels, which has its own zoning hurdles), or reclassification as a second principal residence (zone-dependent, not allowed in most R-1 zones). In practice, almost no Boise homeowner ends up with a larger-than-900 sq ft ADU through the standard path. If you genuinely need more space, the conversation usually shifts to an addition to the main house instead.

How surrounding cities compare

Treasure Valley city caps vary. Meridian allows accessory dwellings up to a percentage of the primary residence's floor area (typically capped around 800-1,000 sq ft on standard lots). Eagle has stricter compatibility rules that effectively limit ADU size to roughly the Boise cap. Each city's unified development code sets the specific number — verify against your jurisdiction during the lot check.

Where to next

If 900 sq ft is enough for your use case, the practical next step is picking a plan: see Boise's six pre-approved plans (all sized to fit the cap), or read up on backyard cottage options for the homeowner framing. For build-side detail, the design-build process page covers what happens from contract to keys. Run feasibility on your parcel via the free lot check.