How Much does it Cost to Convert a Basement into a Legal Adu?

The cost to convert a basement into a legal Accessory Dwelling Unit (ADU) in the Denver metro area varies significantly based on local zoning, square footage, and site-specific structural needs. For 2026 projects, your budget must account for several critical components beyond basic interior finishing.

Key cost factors include:

Permit Fees: In Denver, permit costs typically range from $1,500 to $3,000, while Littleton ranges from $1,200 to $2,500.
Structural Requirements: Denver’s expansive clay soils often require specialized foundation work. This may include helical piers to transfer weight to stable soil or tiebacks to counteract lateral pressure on basement walls.
Safety and Infrastructure: A legal ADU requires egress window installation for emergency escape and natural light. Additionally, the basement plumbing rough-in cost will depend on your existing waste lines and the distance to new kitchen or bathroom fixtures.
Moisture Protection: Depending on the water table, you may need to invest in perimeter drain systems, sump pumps, or moisture and vapor barriers to protect the living space.

Because every basement is unique, we follow a custom-quote approach. Estimates are preliminary and not legally binding until confirmed in writing following an on-site inspection. We recommend consulting a licensed structural engineer for definitive design recommendations to ensure your conversion meets all local building codes and safety standards.

Basement Adu Conversion Cost 2026: Budget Guide

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