Need-to-Knows for Building an ADU in Jefferson County

Accessory Dwelling Units (ADUs) are additional housekeeping units constructed on the same lot as a single-family primary residence. Often called guest houses, parent/in-law units, or carriage houses, they can be separate from the primary dwelling or attached to another building (think garage attic apartment, home addition, or renovated basement).

In Jefferson County, ADUs are limited to having two bedrooms plus full bathroom facilities, a kitchen, and a living area. Beyond adding more living space, ADUs usually increase property values and provide rental income for the property owner.

To help in your initial decision-making, we’ve created this set of need-to-know information for preliminary planning purposes.

Lot Size Requirements

Attached Unit

Minimum Lot Size: 
Per underlying zone district

Maximum ADU Size: 
1200sf or 40% of the primary unit*

Detached Unit

Minimum Lot Size: 
1 acre +

Maximum ADU Size: 
1200sf or 40% of the primary unit*

Minimum Lot Size:
12,500sf  to 1 acre

Maximum ADU Size:
800sf or 40% of the primary unit*

Minimum Lot Size:
7500sf**

Maximum ADU Size:
600sf or 40% of the primary unit*

* Whichever is less
** Detached ADUs not permitted on lots smaller than 7500sf

The Basics

Square Footage and Footprint
Total square footage of all ADUs on the same lot cannot exceed the square footage of the primary dwelling. The ADU footprint cannot exceed 75% of the primary dwelling’s footprint. The ADU must be located within 100 feet of the primary dwelling.

Proximity and Setback
The ADU must be within 100 feet of the primary dwelling on the property.

Detached ADUs must meet setback requirements.

Address
The ADU must have its post-office address.

Owner Lives on Site
When the primary dwelling on the property is a single-family home, the owner must live on the property, either in the residence or the ADU.

Shared Utilities
Whenever feasible, the dwelling’s utilities must be shared with the primary residence.

General Contractor Required
In Jefferson County, licensed general contractors are required for ADU construction.

No Separate Sale
The ADU cannot be sold separately \when the primary dwelling on the property is a single-family home.

Water and Sanitation

For Public Water
Must obtain a letter from the water district approving two dwelling units on the property.

For Well Water
Must obtain a well permit and a letter from the Colorado Division of Water Resources stating that the existing well permit can accommodate a second dwelling unit.

For Public Sanitation
Must obtain a letter from the sanitation district approving two dwelling units on the property.

For Septic Sanitation
Must obtain an active on-site wastewater permit authorizing the total number of bedrooms on the property (including both primary residence and ADU).

Other Requirements/Limitations

Occupancy
No more than three people can occupy the ADU, with no more than six unrelated persons living on the property lot.

Architecture
The architectural style of the ADU must be compatible with that of the primary dwelling.

Parking and Driveway
In addition to parking space for the primary dwelling, one off-street parking spot must be provided for each bedroom in the ADU. Detached ADUs must use the same driveway as the primary dwelling.

Since 2006, Prospect 30 Eight Construction has been building top-quality custom homes, home additions, and Accessory Dwelling Units (ADUs) throughout the Denver Metro Area and the Front Range. If you have any questions, please contact us for a free consultation.