Arvada allows ADUs in all residential zone districts on lots with an existing detached single-family home, as of October 2026. New additions are capped at 40% of the main home's living space and by a lot-size table, while units inside the existing footprint, such as basements, follow a separate limit.
How big can an ADU be in Arvada?
The city's ADU page lists these maximums for new attached or detached units, as of October 2026:
| Lot size | Maximum ADU floor area |
|---|---|
| 6,000 sq ft or less | 600 sq ft |
| 6,001 to 12,499 sq ft | 850 sq ft |
| 12,500 sq ft to 1 acre | 1,000 sq ft |
| Over 1 acre | 1,200 sq ft |
The 40% cap still applies; garages, unfinished basements and patios do not count toward the home's living space. Type B units inside the existing footprint are exempt from the lot-size table and limited to 50% of the home's floor area.
What else does Arvada require?
- Separation. A 1-hour fire separation with a sound rating of STC 45 or higher, and a separate HVAC system.
- Shared services. Water and sewer connect through the main home's service lines; separate services are not allowed.
- Corner lots. Detached units sit at least 10 feet behind the home's front facade on both street sides.
- Covenant. A declaration of covenants from Planning is part of the application.
- Contractor. Only contractors with Arvada Class B and Class C licenses may hold ADU permits, and all subcontractors must be listed.
How long does Arvada take to review?
The city lists about 3 weeks for a detached (Type A) unit and 1 to 2 weeks for a unit within the existing footprint (Type B). Corrections add time. After the permit is issued, the house and ADU are re-addressed as Unit A and Unit B.