Accessory Dwelling Units: Adding Rentable Space to a Property You Already Own

An accessory dwelling unit is a second, self-contained home on a single-family lot. It might be a garage conversion, a basement apartment, an attached addition, or a detached backyard cottage. For owners who already control land, an ADU can add rental income without buying another property.

Why the idea keeps growing

Many cities have loosened rules to address housing shortages, including several in Michigan and Ohio that now allow ADUs in more zones. Lenders and appraisers are also more familiar with them than they were a decade ago. That combination makes the strategy more practical, although rules still vary block by block.

What drives cost

Budgets range widely. The biggest factors are:

• Type. Converting existing space (garage or basement) is usually far cheaper than building a detached unit from scratch.
• Utilities. Extending sewer, water, and electric service, or upgrading a panel, can be a large share of the bill.
• Site work. Slope, access for equipment, and foundation needs.
• Permits and fees. Some cities charge impact or connection fees.
• Finish level. Tenants need durable, simple finishes, not luxury ones.

Get at least two detailed bids and add a contingency of 10 to 20 percent.

How to judge the payoff

Compare the all-in cost with the realistic rent the unit can earn, after its share of taxes, insurance, utilities, and maintenance. A simple yardstick is how many years of net rent it would take to recover the build cost. Also consider whether the unit increases resale value in your area; that depends on local comparable sales, which can be thin.

Three checks before spending money

1. Zoning and code. Confirm the lot allows an ADU, plus size limits, setbacks, parking requirements, and whether the owner must live on site. Call the planning department and ask for the current ordinance.
2. Utilities and structure. Have a contractor or engineer confirm sewer capacity, electrical service, and whether a basement or garage can meet ceiling height, egress, and fire separation rules.
3. Financing. Options include cash, a home equity line, a renovation loan, or a construction loan. Each has different requirements, and some count projected ADU rent while others do not.

Strategic fit

ADUs fit owners who plan to hold long-term, live nearby or have reliable contractors, and want to add a unit without a second mortgage on a second property. They also suit house hacking: live in one unit, rent the other.

Risks to plan for

• Construction delays and cost overruns.
• Permit timelines that stretch for months.
• Shared-lot friction between tenants and the main-house occupants.
• Property tax reassessment after completion.
• Owner-occupancy rules that limit future flexibility.

Section 8 angle

A well-built ADU can qualify for the Housing Choice Voucher program if it meets inspection standards and the rent is reasonable for the area. Check your local housing authority's payment standards for the unit's bedroom count before deciding on the layout.