ADU Cost: Detached, Attached, Garage, and Basement Options
An accessory dwelling unit (ADU) is best understood as a dwelling-function decision, not a universal legal label. Depending on the jurisdiction, a similar space may be called a secondary suite, garden suite, laneway home, accessory apartment, or in-law suite. The name does not establish permission.
What the budget must include
Price the dwelling and the enabling work: site planning, design, approvals, foundation or existing-space preparation, structure, enclosure, kitchen, bathroom, heating and cooling, electrical, plumbing, drainage, entrance, fire and egress measures, access, utility connections, inspections, landscaping restoration, and contingency.
Angi reports a 2026 US market observation of about $110,000 for a garage-to-ADU conversion and about $180,000 for a new ADU. Those are attributed specialist observations, not Canadian prices or universal installed costs. A detached building quote that excludes utilities and sitework is not comparable with an all-in conversion quote.
Compare the physical forms
Detached ADU: Adds a new foundation, structure, roof, utilities, access, drainage, and landscape disturbance. It may offer privacy and a clear separation but can have long service routes and difficult construction access.
Attached ADU: Shares a wall or addition with the main home. It may use existing services more efficiently, but privacy, separation, access, structure, and sound or fire provisions require a site-specific design.
Garage ADU: Reuses a shell but may need slab correction, insulation, new windows, garage-door infill, kitchen and bath plumbing, independent entrance, utility upgrades, and parking analysis. Angi reports detached garage conversions can cost 20–30% more in its market observation because of underground utility work; this is not universal.
Basement or internal suite: Can avoid a new shell but depends on moisture, ceiling height, egress, entrance, structure, separation, ventilation, and service routing. It is not the same budget as ordinary basement finishing.
Above-garage unit: Combines garage structural capacity, stairs, roof work, weather protection, and independent living requirements.
What authorities say—and do not say
CMHC’s homeowner guide describes a sequence of checking property possibilities, zoning and parking, building requirements, a team and budget, plans, approvals, permits, and inspections. The 2024 IRC appendix gives an example model-code framework for internal, attached, or detached ADUs with possible entrance, utility, and scale conditions. Neither is a universal North American rule.
California HCD’s March 2026 handbook is a current example of state-specific ADU law. In Canada, NRC explains that national codes are model documents that take effect through adoption and may be amended by the applicable authority. Verify zoning, building code, parking, fire, egress, accessibility, utility, rental, and inspection requirements locally.
Financing and classification
Fannie Mae’s UAD 3.6 policy illustrates a separate appraisal and mortgage-policy classification of an ADU, including independent living functions. It is not zoning authority and does not guarantee approval, rent, value, or loan eligibility for a project.
Compare complete bids
Ask every bidder to show the unit or conversion scope, sitework, foundation, utility routes and upgrades, access and delivery, kitchen and bath, HVAC, electrical, design and engineering, permits and inspections, fire and egress assumptions, restoration, allowances, and exclusions. Confirm whether a local authority has accepted the intended dwelling classification before paying for detailed plans.
The right ADU budget is the one that distinguishes the physical unit from the property work needed to make it legal, connected, durable, and usable.
A complete ADU comparison
Use the same finished area, kitchen and bathroom level, access, finish quality, utility assumptions, and professional scope for each form. Then compare what is unique:
- detached units add site, foundation, delivery, and longer service routes;
- attached units add tie-in, separation, privacy, and shared-building coordination;
- garage units add shell conversion, parking loss, and slab or utility questions;
- basement units add moisture, height, egress, entrance, and below-grade constraints;
- above-garage units add structural capacity, roof work, stairs, and garage disruption.
Budget for operations, not only construction
Consider service access, filters, meters, shutoffs, heating and cooling, water heating, trash, drainage, parking, and future maintenance. A compact unit with difficult access may be more expensive to service than its construction price suggests. If the owner plans to share systems, write down who pays for replacement and how access is maintained.
Evidence and currentness
Legal and policy examples are date-sensitive. California’s 2026 handbook, British Columbia’s current framework, the IRC appendix, and Fannie Mae policy answer different questions. Use them only for their stated scope. Canadian market evidence is not created by converting US data, and a local authority must decide the actual approval path.
A sensible next step
Start with a property and rules check, then a concept plan, service review, and preliminary budget. Do not pay for detailed construction drawings for a form that cannot meet local site or dwelling requirements. A good early budget narrows the choices without pretending that a national ADU number can replace a local plan.
Separate legal feasibility from financial feasibility
An ADU can be financially attractive but unavailable because of zoning, parking, lot coverage, access, wastewater, fire, or building requirements. It can also be permitted but too expensive once foundation, utility, and sitework are included. Check both questions before ordering detailed drawings.
What belongs in the first comparison
For each form, record finished area, independent functions, entrances, kitchens and bathrooms, shared services, sitework, foundation, structure, design, approvals, inspection, temporary conditions, restoration, and long-term service access. This gives the homeowner a decision framework even when local prices are not yet known.
Do not treat a model-code example, state handbook, Canadian guide, or mortgage policy as a substitute for the local authority. Each source answers a narrower question. A responsible ADU estimate preserves those boundaries.
Separate three questions that are often mixed together
First ask whether the proposed space functions as an independent dwelling for the household’s purpose. Then ask whether the local authority recognizes and permits that configuration. Finally ask whether the property can support it at an acceptable cost. A space can satisfy one question and fail another. An owner’s use of the word “ADU” does not settle zoning, building, appraisal, rental, or utility treatment.
Fannie Mae’s UAD 3.6 policy is an example of appraisal and loan-eligibility classification, not a building-code or zoning decision. The 2024 IRC appendix is a model-code example, not proof of adoption. CMHC, California HCD, and British Columbia materials describe particular Canadian or state frameworks. Use each source for its own question and confirm current local rules before describing a unit as legal, rentable, or approvable.
Model the forms with the same outcome
Use a comparison table for the proposed finished area, sleeping and bathing functions, kitchen, entrance, service arrangement, privacy, parking, access, and finish quality. Then identify the work unique to each form. A detached unit adds site preparation, foundation, delivery, utility routes, drainage, and restoration. A garage unit adds shell adaptation and may sacrifice parking. A basement unit adds moisture, height, egress, entrance, and below-grade constraints. An attached unit adds tie-ins, separation, privacy, and shared-building coordination.
This avoids comparing a building-only prefab price with an all-in conversion estimate. It also reveals when two options do not deliver the same independence. If one form shares a bathroom, entrance, heating system, or laundry, say so rather than presenting it as equivalent to a fully independent dwelling.
Budget the ownership arrangement
Add the operating arrangement to the comparison. A unit may be technically complete yet create ongoing costs for separate meters, shared utilities, internet, insurance, maintenance, snow removal, or repairs to a common driveway. Decide which costs are shared, which are assigned to one household, and how irregular work will be approved. The answer affects both the initial design and the value of the finished unit, especially when the ADU is intended for family use rather than independent rental.
Also test the project against a future change in use. Ask whether the layout, entrances, parking, and utility plan would remain workable if the occupants changed, if privacy became more important, or if the unit later became a rental. This does not require designing for every possible future, but it does prevent a low first estimate from hiding a layout that can serve only one short-term arrangement.
Construction is only the beginning of the financial decision. Record who will maintain the unit, who reaches shutoffs and equipment, how utilities are measured or allocated, where waste and deliveries go, how residents access the property, and what outdoor or parking space is shared. These decisions affect design, service routes, privacy, and future repair costs.
Include an early feasibility stage before detailed drawings: local rule and site check, intended-use definition, utility and wastewater review, concept design, and a total-installed budget. If the site or approval path fails, compare another form before paying for a full construction package. If the path succeeds, request bids that separate building work, property work, professional services, approvals, inspections, allowances, and exclusions.
The useful ADU estimate is a property decision with a unit attached. It explains what the proposed dwelling is, what the local sources do and do not establish, and which unknowns still require local confirmation.