How this calculator works
- The lot comes from the county’s parcel map, with its zoning and jurisdiction.
- The rules come from the city or county code for that zoning district — setbacks, the size limit, lot coverage and where an ADU may sit — each checked against the published code.
- The buildable area is calculated, not estimated: the lot is shrunk by the setbacks, the area in front of the house is removed where the code requires, and the existing buildings are subtracted.
- The plans are real floor plans at their actual size. A plan “fits” only when its whole footprint sits inside the buildable area.
- The flags come from county maps: FEMA flood zones, steep slopes, geologic hazards, septic records and sewer service areas.
A worked example
A 7,000 sq ft lot zoned RM-12 in Grand Junction: a 25-foot front setback for the ADU, 3 feet on the sides and 5 at the rear, and the ADU behind the house. What’s left behind the house is the preliminary buildable area — on a typical lot that’s room for a single-level one- or two-bedroom ADU, or a two-story plan on a narrower strip.
What it doesn’t tell you
- It reads public maps. Easements, utility lines, trees, grade and HOA rules aren’t on them — those are checked on site.
- Building outlines come from open map data and can be missing or out of date.
- Codes change and some sections can be read two ways. A preliminary result is never a permit decision.
Questions people ask
Can I build an ADU on my property?
Usually, if your lot is zoned for single-family homes and there is room after the setbacks. Enter your address above: the tool shows your lot lines, removes the setbacks and the existing house, and tells you which ADU sizes fit in what is left. A builder then confirms easements, utilities and grade on site.
How close to the property line can an ADU be?
It depends on the city and the zoning district. In Grand Junction, a detached ADU follows the accessory-building setbacks — 3 feet from the side lot line and 5 feet from the rear in most residential districts, and 25 feet back from the front. California sets a statewide maximum of 4 feet for side and rear setbacks. The tool applies the rules for your exact zone.
How big can an ADU be?
Each jurisdiction sets a cap. The City of Grand Junction allows up to 900 square feet of habitable space; unincorporated Mesa County allows 900, 1,200 or 1,500 square feet depending on lot size. The tool checks each plan and size against the cap for your lot.
Do I need a survey to know if an ADU fits?
Not to get started. County parcel maps are accurate enough for a preliminary answer, which is what this tool gives you. A builder will usually confirm property corners and easements on site before final plans, and some jurisdictions require a survey with the permit.
Is the result a permit approval?
No. It is a preliminary read of public map data and the published code. Easements, utility lines, trees, slope, HOA rules and local interpretations are confirmed on site before any price or schedule is committed.