A basement ADU is the cheapest square footage you already own
A basement ADU, or accessory dwelling unit, is a self-contained apartment built inside the lower level of a house you already have. Kitchen, bathroom, bedroom, its own way in and out. No excavation, no new roof, no foundation for a second structure. In Southeast Michigan that makes it the least expensive way to add a legal dwelling to a property, and the reason more Washtenaw County homeowners are asking about it every year.
This is one part of our basement remodeling services across Southeast Michigan. See the full range of below-grade work on the basement hub.
Market ranges in the region for a finished basement rental unit run $55,000 to $95,000. The published band for an egress bedroom and in-law suite build sits at $51,000 to $85,000, and a rental unit lands at the top of that or above because it adds a kitchen, a separate entrance and often a second heating zone. A straight basement finishing job with no dwelling requirements runs $30,000 to $75,000, or roughly $24 to $61 per square foot.
Those are market figures for the region rather than a quote. Your number depends on what the basement already has, and the gap between a basement that needs one window cut and a basement that needs drainage, a new panel and a bulkhead is tens of thousands of dollars.
What moves the number
Five things account for most of the spread. Everything else is finish selection.
- The egress opening. A sleeping room below grade needs a code-compliant window or door. Cutting a foundation wall for egress window installation means saw work, a header, a well, backfill and drainage.
- The separate entrance. An interior stair shared with the main house is cheaper. A private exterior door means excavation, a stair or a walkout basement cut, and usually a retaining wall.
- Plumbing elevation. If the sanitary line leaves above the slab, the basement bathroom needs an ejector pit. That is concrete cutting, not fixture shopping.
- Water history. Any evidence of seepage gets solved before framing. Waterproofing adds $5 to $10 per square foot and it is not optional here.
- Mechanical capacity. A second dwelling wants its own heating and cooling, and older panels rarely have room for a range, a dryer and a mini split.
The basement finishing cost calculator will get you inside a reasonable band in about four minutes, and our writeup on basement finishing costs in Michigan breaks the line items down further.
What the code actually requires below grade
This is where most basement apartment plans get expensive, and where a lot of unpermitted ones fail an inspection later. Michigan’s residential code sets a hard floor for habitable space, and a rental unit has to clear all of it.
Ceiling height is at least seven feet across half the required floor area. That single line kills more basement plans than any other, because ductwork and beams eat headroom fast, and a soffit that solves one problem can drop a room below the minimum. We measure to the bottom of the lowest obstruction, not to the joists, before anything gets drawn.
Every sleeping room needs an egress opening with 5.7 square feet of net clear area, at least 24 inches of clear height and 20 inches of clear width, with the sill no more than 44 inches above the finished floor. A window well has to project 36 inches from the foundation and gets a permanent ladder if it is deeper than 44 inches below grade. Bedrooms need 70 square feet with no dimension under seven feet, though a room that small will not rent well.
Smoke alarms are interconnected, hardwired and battery backed, inside the bedroom, outside it and on every level. Carbon monoxide detection goes within 20 feet of any sleeping room when fuel-burning equipment is present, which in a Michigan basement is almost always. Our writeup on Michigan basement bedroom requirements covers the inspection sequence in detail, and the basement permits guide covers who issues what.
Zoning is the part that varies by street
Michigan has no statewide ADU law. The Michigan Zoning Enabling Act leaves it to each municipality, so the answer changes at the city line. Two houses four miles apart can get two different answers, and the zoning question has to be settled before anyone spends money on drawings.
Ann Arbor expanded its ADU rules in 2021 and now permits them on lots with one single-family home across a wider set of districts, including R3 and the R4 multiple-family districts. Interior conversions count, and the city names basements specifically. Size caps generally land between 600 and 800 square feet depending on the lot, owner occupancy is required, and short-term rental is off the table. A building permit is required, but no site plan, public hearing or public notice.
Elsewhere in Washtenaw County the townships differ, and some allow accessory dwellings only for a caretaker or a family member. In Ypsilanti, where our shop is, the accessory-structure and site regulations are the starting point rather than a dedicated ADU chapter. Our guide to accessory dwelling units in Michigan and the companion piece on planning a basement rental suite go community by community.
We make the zoning call the first task, not the last. If the district does not allow a second dwelling, the same basement can still become a guest suite or a basement apartment for family use, which changes the permit path and usually the budget.
The landlord questions nobody asks until later
The construction is the part people plan for. The operating decisions are the part that surprises them, and several of them change the drawings, so they belong in design rather than in the first month of a lease.
Utilities come first. One meter means the rent has to carry the tenant’s usage, and a hot August or a cold February moves that number more than people expect. Separating electrical service is possible on many houses and costs real money, and the utility has the final say on whether a second drop is even available at your address. Water is almost never worth separating on a basement unit.
Then the paperwork. Several communities in our service area require rental registration and a periodic inspection, and a homeowner’s policy usually needs a landlord endorsement or a different policy entirely once you take rent. Your mortgage and your insurer both want to know. None of that is our department, and all of it is cheaper to learn before framing than after a tenant moves in.
Financing is the last piece. A lower-level dwelling that produces income sometimes qualifies differently than a straight remodel, so it is worth looking at renovation financing early. Our overview of a basement apartment conversion in Michigan covers the sequence, and a consultation is where we sort out which of these apply to your address.
Timeline and how much of your house you lose
Design and permitting run four to eight weeks in most Washtenaw and Wayne County jurisdictions, longer if a variance or an engineered drawing is involved. Construction runs eight to fourteen weeks for a one-bedroom unit. Cutting the egress opening and the exterior door is the loudest week, and it happens early.
You keep your kitchen and your bathrooms the whole time. What you lose is basement storage, the laundry for a few days while the laundry room is rebuilt or relocated, and quiet during demolition and concrete work. Water and power get shut off in short scheduled windows rather than for days. Interior basement work runs through a Michigan winter without a weather delay, which is why our lower-level schedule fills between November and March. The exterior door and the excavation are the parts that want unfrozen ground.
What is in the price and what is not
A Wright’s basement rental unit budget includes design and permit drawings, permit fees and inspections, framing, insulation and soundproofing, rough and finish electrical, plumbing and mechanical, drywall, trim and paint.
It also includes basement flooring, lighting, a kitchenette or full kitchen, the bathroom, the egress opening, and daily photo updates so you can see the work without standing in it. If a second bedroom is in scope, the basement bedroom page covers what a smaller sleeping room needs to clear.
It does not include appliances beyond what the drawings specify, furniture, landscape restoration past backfill and grade, a service upgrade if the utility requires a new drop, or remediation for problems we cannot see until demolition. If we find water damage, a failed tile line or something that needs mold remediation, that comes to you as a written change order with the cost and the schedule effect before anyone touches it. Structural surprises route through foundation repair the same way.
How it goes wrong here, and what to look at first
Southeast Michigan basements fail in predictable ways, and a rental unit punishes every one of them because someone will be sleeping down there every night rather than doing laundry twice a week.
Clay soil holds water against the foundation, and spring melt plus a heavy rain is when a basement that seemed dry for a decade shows itself. Look for a chalky white line on the block, rust at the base of steel posts, a sump that runs in bursts in April, and a musty smell that comes back after you air the space out. Any one of those means the water problem gets solved before framing, because drywall over a wet wall is a two-year clock.
Then check the boring things. Where does the sanitary line leave the house, above the slab or below it. How many spare breaker spaces are in the panel. Is the dryer vent path shorter than the manufacturer’s maximum. Does the furnace have the capacity for a second zone, or is a separate system cheaper than reworking the trunk. Are the ceiling options compatible with seven feet of clear height once the ducts are boxed.
Radon belongs on that list too. Testing costs almost nothing measured against the EPA action level of four picocuries per liter, and a mitigation system installed during construction is a fraction of what it costs to retrofit around finished walls. We answer those questions on the first visit, and the answers usually reset the budget before a drawing exists.
Questions we get asked
Will a basement ADU appraise or just cost money
Below-grade square footage rarely counts the way above-grade space does in an appraisal, so treat rent and family use as the return rather than a dollar-for-dollar gain in value. A finished lower level with a legal bedroom and a full bath still moves a house faster in this market than an unfinished one.
Can I rent it out short term
In Ann Arbor, no. Short-term rental of an ADU is prohibited and owner occupancy of one of the two units is required. Other communities in the region treat it differently, and a few have no framework at all, which is its own kind of risk. Get the answer in writing from the municipality before you count on the income.
Do I need a separate entrance
Not always for a permit, but usually for a tenant. Shared interior access works for family. Anyone paying rent wants a door of their own, and it changes how the space lives more than any finish decision.
What if my ceilings are under seven feet
Then a rental unit is off the table without lowering the floor, which is excavation and underpinning and rarely pencils out. The honest answer at that point is a different project. We will tell you on the first visit rather than after you have paid for drawings.
Start with a measurement, not a rendering
We are a design-build contractor, so the drawings, the permits and the crews are all ours. One contract, one schedule, one phone number when something needs answering. See how a project runs from first visit to final inspection, or view our portfolio to see finished lower levels across the six counties we serve.
Contact us now for a free quote, or call the shop at (734) 540-0347. Bring us your address and we will tell you what your zoning district allows before we talk about finishes. Request an estimate and we will get the tape measure into the basement.
