Under the same roof, behind their own front door.

An addition adds rooms. An ADU adds a dwelling.

The difference is not size and it is not where it attaches. It is whether the new space is an independent dwelling: its own entrance, its own kitchen, its own bathroom, and the ability to be occupied without walking through the main house.

That distinction decides which rules you are assessed under, and it is worth getting right early. Built as an addition, you are in the ordinary addition process. Built as an ADU, you get the state’s administrative approval, the objective-standards test, and the protection that comes with them — and you get a second dwelling on the title rather than a bigger house. We built one as a Denver addition with its own kitchen, bedroom and bath.

Bulk plane, and what your town calls this.

Two constraints do most of the shaping. Wheat Ridge and Englewood both apply bulk plane — an angled envelope above your setbacks that limits how tall you can build close to a boundary. On an attached unit, which by definition sits against an existing wall, bulk plane frequently decides whether the second storey works at all. Centennial adds a design-match rule: the unit must match the principal dwelling on at least three of roof pitch, cladding, windows, doors, architectural features and colour palette.

Then there is what your town calls it. Cherry Hills Village regulates this as a “guest house”, permitted in R-1 and R-2 only, and its published handout limits occupancy to family members or non-paying guests of the permanent occupants — which is difficult to square with the state ADU law, and may simply be a stale document. We confirm the current position before design rather than repeating a handout at you.

Build for the second occupant, not just the first.

A unit built precisely around one parent’s needs today is a unit that is hard to let, hard to sell and hard to repurpose in five years. The ones that work long-term are built as proper independent dwellings that happen to be occupied by family first — level access and a wider door because it costs almost nothing at framing stage and everything later, and a layout that reads as a home rather than as an annexe.

That is the difference between an expense and an asset on your title.

• Feasibility and zoning check before design spend
• Structural design for the connection to the existing house
• Bulk plane and design-match assessment for your jurisdiction
• Separate entrance, kitchen and bathroom to dwelling standard
• Level access and door widths designed in, not added later
• Administrative permit submittal and inspections
• One contract, one accountable team

Attached ADU questions we get asked.