Accessory dwelling unit (ADU) permits

What an ADU is, and why permits are more involved

An accessory dwelling unit, also called a granny flat, accessory apartment, in-law suite, backyard cottage, or second unit, is a complete, independent home on the same lot as a main house. HUD’s case study on ADUs described them as “an inexpensive way to increase their housing supply” for communities facing housing shortages.

ADUs come in three common forms:

• Detached: a separate small house or a unit over a detached garage.
• Attached: an addition to the main house with its own entrance.
• Interior or conversion: a basement, attic, or garage converted into a unit.

Because an ADU is a dwelling, none of the model code’s permit exemptions apply. The International Residential Code’s exemption for small accessory buildings (R105.2) covers only one-story detached structures up to 200 square feet that are used as accessory structures, not as places to live. See our shed permit guide.

What the model code says

Construction of an ADU is regulated like any other house or addition: foundations, framing, fire safety, egress, insulation and energy, plumbing, mechanical, and electrical. Specific points that come up with ADUs:

• The 2024 IRC ADU appendix. ICC’s 2024 IRC includes Appendix BC, which “provides for the design and construction of accessory dwelling units (ADUs), an alternative to two- and multiple-family residential construction that promotes increased housing supply and affordability.” Appendices are optional and take effect only if your state or local government adopts them, so ask whether yours has.
• Tiny houses. Appendix BB of the 2024 IRC gives relaxed provisions “for dwelling units that are 400 square feet (37 m2) or less in floor area, excluding lofts,” mainly for loft ceiling heights and compact stairs and ladders. It is also optional.
• Fire separation. An attached or interior ADU may need fire-resistant separation from the main house. How much depends on whether your jurisdiction treats the pair as one dwelling with an accessory unit or as a two-family dwelling, which is a key reason to confirm early which provisions apply.
• Egress, smoke and CO alarms, and energy. Bedrooms need emergency escape openings, and the unit needs its own alarms and must meet the energy code for new or converted space.
• Utilities. Separate or shared water, sewer, gas, and electrical services, sized for two households. Utility providers may have their own connection rules and fees.

Zoning and state ADU laws

Zoning is usually the first and biggest hurdle. Local ordinances commonly regulate:

• where ADUs are allowed (zoning districts, lot size);
• size (maximum floor area, or a percentage of the main house);
• height and setbacks;
• parking;
• owner occupancy;
• short-term rental restrictions;
• design standards and number of units per lot.

A growing number of states limit how strict these local rules can be. Examples we verified in state statutes:

These laws have scope limits and exceptions, so read the statute and your local ordinance together. Where our state research covers ADU rules, they appear in the state notes below.

HOAs may have covenants on additional units or rentals. Check whether your state’s ADU law addresses private covenants.

Permits and inspections you can expect

1. Pre-application or zoning check. Confirm that an ADU is allowed and which standards apply. Many cities publish ADU checklists.
2. Zoning or planning approval, sometimes administrative, sometimes with notice to neighbors, depending on state law and local rules.
3. Building permit plan review of architectural, structural, and energy documents.
4. Trade permits for electrical, plumbing, and mechanical work, and often gas.
5. Utility and septic approvals: water and sewer connections, or a health department permit for septic changes.
6. Inspections throughout construction: foundation, framing, rough trades, insulation, and final.
7. Certificate of occupancy or final approval before anyone moves in.
8. Address assignment for the new unit, often through the county or city addressing office.

What to prepare

• Site plan showing both units, setbacks, parking, utilities, easements, and septic components.
• Floor plans and elevations, including egress windows and heights.
• Structural and energy compliance documents.
• Utility plan: meter strategy, panel capacity, and water and sewer connections.
• Septic evaluation if not on public sewer.
• Ownership and occupancy documents if your jurisdiction requires a covenant or affidavit.
• HOA review if covenants apply.

How permit fees are calculated

ADU costs often include several government fees, each set locally:

• Building permit and plan review fees, commonly based on valuation or square footage. Jurisdictions often benchmark valuation against the International Code Council’s Building Valuation Data. The 2024 IRC’s optional Appendix AB “provides guidance to jurisdictions for setting appropriate permit fees.”
• Trade permit fees for electrical, plumbing, and mechanical work.
• Zoning or planning application fees.
• Impact, capacity, or connection fees for water, sewer, schools, parks, or transportation. These can be among the largest items.
• Septic permit fees from the health department.

Ask your permit office and utility providers for fee estimates early.

Common mistakes

• Designing before checking zoning, then finding out the lot is too small, the height is too great, or parking is required.
• Assuming a shed or garage can become living space without a change-of-use permit.
• Underestimating utilities: panel upgrades, sewer laterals, and septic capacity.
• Forgetting fire separation for attached and interior units.
• Renting before final approval, or using the unit as a short-term rental where it’s prohibited.
• Missing impact or connection fees in the budget.