Idaho ADU Laws and Regulations
Idaho does not have a statewide ADU law, so regulations are set at the city and county level. Most Eastern Idaho cities have adopted ADU-friendly policies in response to housing shortages, but the specifics vary significantly. Here is what you need to know for the major markets.
Rexburg
ADU-FriendlyRexburg has some of the most permissive ADU regulations in Idaho, driven by housing demand from BYU-Idaho. Basement apartments are common and generally permitted in residential zones. Owner occupancy of the primary unit is typically required. The city has a streamlined permitting process due to high volume.
Rental income: $600–$900/mo
Idaho Falls
Permitted with ConditionsIdaho Falls allows ADUs in most residential zones with a conditional use permit. The property must meet parking requirements (usually one additional off-street space), and the ADU must be clearly subordinate to the primary dwelling. Owner occupancy requirements apply.
Rental income: $700–$1,100/mo
Pocatello
Permitted with ConditionsPocatello permits ADUs in several residential zones. The application process includes a site plan review. ADUs must comply with the city's building and safety codes, including fire separation, egress, and parking requirements.
Rental income: $550–$850/mo
For detailed building code requirements including egress, ceiling height, and fire safety, see our building codes guide. City-specific information is available in our Rexburg, Idaho Falls, and Pocatello guides.
Separate Entrance Requirements and Options
A separate entrance is the defining feature of a basement apartment that distinguishes it from a finished basement with extra rooms. Most Idaho jurisdictions require an independent exterior entrance for a legal ADU — tenants should not have to walk through your living space to access theirs.
The most common approach is a concrete stairwell with a landing and exterior door. This involves excavating alongside the foundation, pouring concrete walls and steps, installing a weatherproof exterior door, and adding a drain at the bottom of the stairwell to prevent flooding. In Eastern Idaho, this typically costs $4,000 to $8,000.
If your lot has a slope — common in the foothills around Pocatello and Sun Valley — you may be able to add a walk-out entrance at grade level. This is the easiest and cheapest option, requiring only a new door opening cut through the foundation wall, a door frame, and a small landing or patio. Cost: $2,000 to $4,000.
Whichever entrance type you choose, proper drainage is critical. Idaho's spring snowmelt and occasional heavy rains can flood below-grade stairwells if drainage is inadequate. A channel drain at the base connected to a sump pump or daylight drain is essential. See our waterproofing guide for details on drainage systems.
Kitchenette Requirements and Design
A legal ADU must have kitchen facilities for independent living. Idaho building code requires a minimum of a sink with hot and cold water, a cooking appliance, and a refrigerator. A full kitchen is not required — a kitchenette satisfies code and is more practical for the available space.
A typical basement apartment kitchenette includes a compact sink, 18 to 24-inch dishwasher (optional), apartment-size refrigerator, microwave, and a two-burner cooktop or range. A vent hood or microwave with exhaust fan is required if a cooktop is installed. Plan for 6 to 10 linear feet of counter and cabinet space. See our bar and kitchenette guide for plumbing and countertop details.
Budget $5,000 to $10,000 for a complete kitchenette installation including plumbing, electrical, cabinetry, countertops, and appliances. Using the existing plumbing rough-ins (if available) saves $1,500 to $3,000 on this portion.
Rental Income Potential in Eastern Idaho
Eastern Idaho has a housing shortage that drives strong rental demand, particularly in college towns. Here is what you can expect for monthly rental income from a basement apartment:
| City | 1-Bedroom | 2-Bedroom | Demand |
|---|---|---|---|
| Rexburg | $600–$900 | $800–$1,200 | Very High (BYU-Idaho) |
| Idaho Falls | $700–$1,100 | $900–$1,400 | High |
| Pocatello | $550–$850 | $700–$1,100 | High (ISU) |
| Blackfoot | $500–$750 | $650–$950 | Moderate |
| Rigby | $600–$850 | $750–$1,050 | Moderate-High |
At $800 per month average rent, a $45,000 basement apartment investment pays for itself in under 5 years. The ongoing income stream, combined with the added property value, makes this one of the highest-ROI home improvements available in Idaho. For full cost analysis, see our cost guide and financing options.
Basement Apartment FAQ
Are basement apartments legal in Idaho?
Yes, but regulations vary by city. Idaho Falls, Rexburg, Pocatello, and many other Idaho cities allow accessory dwelling units (ADUs) including basement apartments, though each city has specific zoning requirements, permitting processes, and occupancy standards. Always check with your local planning and zoning department before starting construction.
How much rental income can a basement apartment generate?
In Rexburg near BYU-Idaho, a 1-bedroom basement apartment rents for $600 to $900 per month. In Idaho Falls, expect $700 to $1,100. Pocatello ranges from $550 to $850. These numbers can cover a significant portion of your mortgage payment, making a basement apartment one of the best investments in Idaho real estate.
How much does it cost to build a basement apartment?
A complete basement apartment with separate entrance, kitchenette, bathroom, bedroom, and living area costs $35,000 to $65,000 in Eastern Idaho. The biggest cost drivers are plumbing (kitchenette and bathroom), the exterior entrance, and meeting fire separation requirements. If plumbing rough-ins already exist, costs can drop to $25,000 to $45,000.
Do I need a separate entrance for a basement apartment?
Most Idaho cities require a separate exterior entrance for a legal ADU. This is both a safety requirement (fire egress) and a practical necessity for tenant privacy. A new exterior entrance with a concrete stairwell typically costs $3,000 to $8,000 depending on grading and construction complexity.
What are the fire separation requirements?
Idaho building code requires a 1-hour fire-rated separation between the main dwelling and the ADU. This means 5/8-inch Type X drywall on the ceiling and shared walls, fire-rated doors between units, and interconnected smoke and CO detectors. These requirements protect both the homeowner and the tenant.
Related Guides
Make the property tenant-ready inside and out. Visit Idaho Sprinkler Systems and Idaho Yard Pros for curb appeal and landscaping.