How long does it take to build an ADU in Boise?
5-7 months contract-to-keys is the typical Boise ADU timeline on a pre-approved plan. 7-9 months on custom. Permits typically clear in 3-5 weeks for pre-approved, 8-12 weeks for custom. Construction itself runs 4-5 months across both paths — the difference is entirely at the city's planning office. Weather rarely delays a Treasure Valley build; we frame and finish year-round outside the occasional sub-20°F cold snap.
Stage-by-stage timeline
| Stage | Pre-approved | Custom |
|---|---|---|
| Lot check + initial scope | 1-2 weeks | 1-2 weeks |
| Plan selection + contract | 1-2 weeks | — |
| Custom design | — | 4-8 weeks |
| Permit submittal + review | 3-5 weeks | 8-12 weeks |
| Site work + foundation | 3-4 weeks | 3-4 weeks |
| Framing + enclosure | 4-6 weeks | 4-6 weeks |
| MEP rough-in + insulation | 2-3 weeks | 2-3 weeks |
| Drywall + finish work | 4-6 weeks | 4-6 weeks |
| Final inspections + C of O | 1-2 weeks | 1-2 weeks |
| TOTAL | 20-30 weeks (5-7 mo) | 29-43 weeks (7-9 mo) |
Where most projects actually slip
Plan-checker comments
Custom plans get redlined more than pre-approved. Each round of comments adds 1-3 weeks depending on how quickly your design team responds. Pre-approved skips this entirely since the city has already reviewed the structural set.
Utility connection scheduling
Idaho Power has its own scheduling for new service drops. Sewer and water connections through the City of Boise are predictable; gas through Intermountain Gas takes 2-4 weeks. Most slips come from Idaho Power requiring a transformer upgrade at permit — adds 4-8 weeks if it triggers.
Material lead times
Cabinets, windows, and finish appliances have 6-12 week lead times depending on selection. We order early and stage materials on-site to avoid finish-phase delays. Custom cabinetry from specialty shops can add 2-4 weeks vs stock options.
Homeowner decisions
The fastest builds happen when the homeowner makes finish-phase decisions (paint, tile, fixtures, hardware) on schedule. The slowest happen when decisions stall at the selection stage. We send a decision schedule at contract signing — projects that follow it finish on time; projects that don't add 2-6 weeks at finish.
Faster paths
- Pre-approved plan instead of custom (saves 4-8 weeks at the city)
- Standard finish instead of Premium (faster material lead times)
- Garage conversion instead of detached (saves 1-2 months total)
- Pre-decide finish selections at contract signing
- Site with utilities at the curb and no transformer upgrade needed
Seasonal considerations in the Treasure Valley
Treasure Valley winters are mild enough to frame and finish through. Spring and early summer are the busiest scheduling windows — plan check queues are longer and trade availability tighter. Fall and winter starts often finish faster because the city office and trade calendar both run lighter. There's no construction season here the way there is in Idaho's mountain markets.
How surrounding cities affect timeline
Permit review times vary by jurisdiction. Boise: 3-5 weeks pre-approved, 8-12 custom. Meridian: typically 3-4 weeks pre-approved, 6-10 custom. Eagle: often longer on custom due to architectural compatibility review (10-14 weeks). Garden City, Nampa, Caldwell each track close to Boise's window.
Where to next
Timeline depends on plan path and builder model. See Boise's six pre-approved plans for the 4–8 week permit shortcut, the design-build contractor overview for end-to-end coordination, or the ADU builder page for how we run a build. Run feasibility on your parcel via the free lot check.