Remodeling services
ADU builders: ask planning first
The four ADU types, what separates an ADU from an addition, the work in order, and what a written proposal must name. Plan your remodel: tell us the project and a person replies.
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Written by HyreRemodel Research Desk Primary-source research and fact checking
The short answer
ADU builders put up an accessory dwelling unit (ADU): a self-contained second home on a lot that already has one, with its own kitchen, bathroom, sleeping area and entrance.
Because it is a second dwelling rather than more of the same house, it answers to different local rules and needs its own utility and fire-separation provisions.
Settle this before anything else
Ask your city or county planning department what it allows on your parcel today, before paying for drawings: size, height, setbacks, parking, owner-occupancy and renting. ADU rules are local and have changed recently in several states. Converting part of a house built before 1978 is lead-safe work under EPA rules.
What makes an ADU a different project from an addition?
A kitchen of its own is what can turn a suite into a separate unit, and a separate unit brings its own set of rules.
Size, height, setbacks, parking and owner-occupancy are set by your city or county within state law, and several states have changed that law recently.
Sewer or septic capacity, the electrical service and metering decide projects before the design does.
Plan your accessory dwelling unit
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When is an ADU the right project rather than an addition?
The two get mixed up, and they solve different problems.
Their own entrance, kitchen and bathroom, for a parent, an adult child or a tenant. If shared living works, an addition is the simpler project.
That needs the unit to be legally rentable where you live, a separate question from whether it may be built.
A garage, a basement with its own door, or an outbuilding already has walls and a roof.
Within the setbacks and coverage limits, if a detached unit is what you want.
A parent now and a tenant later, or the other way round.
An ADU is a permanent second dwelling with lasting tax and insurance consequences.
In what order does an ADU get built?
The first three steps are paperwork and feasibility, and they can take longer than the construction.
What this parcel allows today: unit size, height, setbacks, parking, entrance location, owner-occupancy and renting. Free, decisive, and the step people skip.
Whether the sewer connection or septic system can take a second dwelling, and whether the water and electrical services can carry it. Septic capacity can stop a project outright.
A small dwelling has to fit everything into a fixed size cap. This is where a designer earns the fee.
An ADU is permitted work, and stamped drawings can be required. Some jurisdictions have faster review for ADUs; ask what yours takes.
A foundation for a new unit, or reframing and upgrading the structure being converted. A garage slab can need insulating and leveling.
Water, sewer, gas and electrical to the unit, trenched across the lot for a detached build. Separate or shared metering is decided here.
Wiring, plumbing and ducts, plus the rated separation between dwellings that attached and interior units need.
A second dwelling is inspected as one: expect the full sequence, not a remodeling subset.
Insulation, drywall, kitchen, bath and finishes, then meters set, the final inspection and approval to occupy.
A second dwelling changes both. Tell your insurer, and ask the assessor what the unit adds, before anyone moves in.
Which of the four ADU types fits your lot?
The more structure you build from scratch, the more each square foot costs.
| Type | What it involves |
|---|---|
| Interior conversion | Part of the house, such as a basement or lower level, given its own entrance, kitchen and bathroom. The same ceiling-height and escape-opening rules apply as for any below-grade room. |
| Garage conversion | The foundation, walls and roof already exist. The work is insulating the shell and slab, framing in the door opening and, where the city requires it, finding replacement parking. |
| Attached new build | A new unit built onto the house. The shared wall saves some structure but brings fire-separation and sound rules between dwellings that an addition would not face. |
| Detached new build | A separate small building: foundation, shell, roof, and utility runs across the lot. The costliest per square foot, and the most private. |
What belongs in a written ADU proposal?
Two proposals for the same size unit can describe different jobs. These lines make them comparable.
The size, height, setbacks and parking the design assumes, matched to what the planning department confirmed.
Sewer or septic work, water, gas and electrical runs, and whether the unit is separately metered.
The rated assembly between dwellings, for attached and interior units.
For a garage or basement: slab, insulation, vapor control, the door opening and escape windows.
Rot, old wiring, a cracked slab: how it is reported, priced and approved before work goes on.
Permit, impact and utility connection fees, and which are in the price.
Who pulls the permit and books inspections, and every change signed with its price before the work.
Whether the unit has its own system or shares the house's, and whether that system has the capacity.
Where are the safety lines in an ADU?
Settle these before design is final. Each one belongs to a certified firm, a licensed trade or your building department.
Two homes sharing a structure need a rated wall or floor between them under the code your jurisdiction adopted.
Each sleeping room needs an escape opening (R310), and habitable rooms need at least 7 ft 0 in of ceiling height (R305). Basements and garages can fall short.
Converting part of a pre-1978 house disturbs paint: past 6 sq ft per room inside, the firm must be EPA Lead-Safe Certified. Test old flooring and insulation for asbestos first.
New gas lines and service changes are licensed, permitted work. Call 811 before trenching utilities across the lot.
Code references are to the 2021 International Residential Code. Your building department's adopted edition and local amendments decide what applies.
What moves the price of an ADU?
No figures here, by policy. The utility lines are the ones people fail to see coming.
| Factor | Why it changes the number |
|---|---|
| Conversion or new build | An existing garage or basement brings a foundation, walls and roof. A new build pays for all three. |
| Sewer or septic | A second dwelling adds load. A sewer connection may need upgrading; a septic system may need enlarging or replacing, which can decide whether the project goes ahead. |
| Electrical service and meters | Whether the service carries a second home, and whether the unit gets its own meter, which costs more up front and makes renting simpler. |
| Distance from the house | For a detached unit, every foot of trench for water, sewer, gas and power is cost, and the sewer line needs slope. |
| The kitchen | A full kitchen brings a kitchen's trades and costs into a small building. |
| Fire separation and sound | Rated assemblies between dwellings, which an addition of the same size does not need. |
| Parking | Where replacement parking is required, it can mean paving, a curb cut and lost yard, or it can stop the project. |
| Fees | Some jurisdictions charge impact and connection fees on a new dwelling; some waive or cut them for ADUs. Ask. |
| Design and engineering | Stamped drawings can be required, and small does not mean simple: design costs more per square foot on a small building. |
HyreRemodel publishes no national average price for this work. Size, finish level, whether plumbing, wiring or walls move, the age of the house and regional labor rates move it far enough that a single figure would mislead.
May you build it, and may you rent it?
Those are two separate permissions. A jurisdiction can allow the first and restrict the second through owner-occupancy rules, short-term rental limits or permit conditions. If rent is part of the case for building, get the rental rules confirmed in writing before you commit money.
What to ask before you sign
What to have ready before the first meeting
And the plat or survey, if you have one.
In writing where you can get them.
Which one serves the lot, and the septic system's age and size if it has one.
The electrical panel label and the main service.
And whether that is likely to change within ten years.
Confirmed, if the numbers depend on rent.
Where this goes wrong
ADU rules are local, specific and changing. Drawings made against assumed rules get redrawn. The inquiry is free.
They are separate permissions. A unit built on rent that turns out to be restricted is an expensive guest room.
A system sized for one home now serves two. Enlarging or replacing it can end the project, which is better known in week one.
An uninsulated slab and walls, no vapor control and a wide door opening: bringing it to habitable standards is most of the work.
Property tax and insurance can change, and a rented unit brings landlord duties. None of it shows in a construction budget.
A size limit is a ceiling, not a target. A slightly smaller unit that is well planned lives and rents better.
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Questions
What makes something an ADU rather than an addition?
Why does this page not list size limits or setbacks?
Is a garage conversion cheaper than a new ADU?
Can I rent out an ADU?
Will an ADU raise my property taxes?
Do I need separate utility meters?
What is fire separation, and why does an ADU need it?
How long does an ADU take?
Does HyreRemodel build ADUs?
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