For a 400 sq ft garage conversion ADU, build the budget as a trade-by-trade scope: design and permits, slab moisture testing and sealing, garage door removal and wall framing, windows and egress, electrical, kitchenette plumbing, bathroom plumbing if included, insulation and drywall, mini-split HVAC, finishes, and contingency. Do not assume the existing garage is habitable. The slab, former exterior openings, drainage, ceiling height, fire separation, and utility capacity must be inspected first. Exact dollar amounts depend heavily on location, permit requirements, access, existing conditions, and finish selections, so use local contractor bids rather than a national blanket price.
Build a realistic budget before choosing finishes
A garage conversion ADU budget should begin with the work required to make the space legal, dry, insulated, ventilated, safe, and serviceable. Cosmetic finishes come later. A garage that looks finished may still have a slab that allows moisture migration, an uninsulated overhead door, insufficient electrical capacity, no code-compliant emergency escape opening, or plumbing that cannot be extended economically.
For a 400 sq ft studio, request separate allowances or bids for each major scope rather than one unexplained lump sum. This makes it easier to compare contractors and identify omissions.
| Budget section | What to include | What changes the cost |
|---|---|---|
| Preconstruction | Measured drawings, design, engineering when needed, permit fees, inspections, and site investigation | Local review process, structural changes, zoning conditions, and existing documentation |
| Existing-condition correction | Moisture testing, slab treatment, drainage corrections, demolition, and hazardous-material evaluation when applicable | Water entry, cracks, low floor elevations, prior coatings, and concealed damage |
| Building envelope | Garage door removal, wall framing, insulation, weather barrier, windows, exterior cladding, and air sealing | Wall thickness, structural header needs, siding match, window count, and fire separation |
| Utilities | Electrical service and circuits, kitchenette plumbing, bathroom plumbing if included, water heating, and HVAC | Distance to connection points, panel capacity, trenching, drain slope, and local utility rules |
| Interior completion | Drywall, flooring, cabinets, counters, fixtures, paint, trim, and appliances | Durability, custom work, accessibility, and finish selections |
| Closeout | Testing, corrections, final inspections, cleaning, and documentation | Inspection comments and incomplete subcontractor work |
Use a written allowance for every item that has not been selected. A contractor quote should state whether permit fees, appliances, utility charges, testing, engineering, finish materials, and tax are included. Keep a separate contingency for concealed conditions rather than treating it as free money for upgrades.
Check the existing garage before pricing the conversion
Have the slab, walls, roof framing, openings, drainage, electrical service, and plumbing routes evaluated before accepting a conversion price. A low bid based on the assumption that everything can remain may increase after demolition.
Confirm the following conditions:
- The floor is reasonably level and does not show active water entry, significant settlement, or persistent dampness.
- The ceiling and wall assemblies can be insulated without creating moisture or headroom problems.
- The garage is not relying on an unprotected connection to the house for fire separation.
- The electrical service and panel can support the new dwelling loads.
- Water and drain connections can be reached without excessive slab cutting or an unworkable drain slope.
- New windows and doors can meet the local requirements for light, ventilation, safety glazing, and emergency escape where applicable.
Because an attached garage can be close to a property line, review the applicable attached garage setback and firewall rules before changing exterior walls or openings. The linked guidance is a starting point, not a substitute for confirmation with the local building department.
Budget for concrete slab moisture sealing
The existing garage slab is not automatically suitable as a finished ADU floor. Concrete can transmit water vapor even when there is no visible puddle. Floor coverings, adhesives, and wall finishes may fail if the slab has excessive moisture or if groundwater is entering through cracks and joints.
A sensible slab scope may include:
- Inspect cracks, joints, floor-wall edges, stains, and low areas.
- Test for moisture using a method accepted by the flooring or coating manufacturer.
- Correct active water entry or drainage problems before applying a sealer.
- Prepare the surface by removing contaminants, loose material, and incompatible coatings.
- Repair cracks and joints with products suitable for the movement and moisture condition.
- Apply a moisture-mitigation system compatible with the planned flooring and wall construction.
- Verify the cured surface before installing flooring, adhesive, insulation, or partitions.
Do not describe a generic concrete sealer as a universal vapor barrier. Product performance, surface preparation, allowable moisture level, cure time, and installation conditions are manufacturer-specific. If the slab must be raised, leveled, excavated, or replaced, the scope becomes substantially different from simple sealing.
For background on slab work and the variables that affect concrete construction pricing, see the guide to concrete slab costs for garage expansions. A conversion may require repair or mitigation rather than a new slab, but the same site and access issues can affect both scopes.
Replace the garage door opening correctly
Removing the overhead garage door is one of the most important envelope changes in the project. The opening must become a weather-resistant, insulated wall assembly, usually with one or more windows or a new exterior door if the design requires it.
The budget should identify:
- Demolition and disposal of the door, tracks, operator, and related trim.
- Framing infill sized for the opening and connected to the existing structure.
- Any header, post, footing, or structural review needed for the new wall.
- Exterior sheathing, weather-resistive barrier, flashing, siding, and sealants.
- Insulation and interior air sealing.
- Windows or doors, including flashing, trim, hardware, and safety glazing where applicable.
- Interior drywall and finish repairs around the former opening.
Do not assume the existing garage door header can simply remain exposed or be cut without review. The final framing method depends on how the opening was built, the roof and wall loads, and the local structural requirements. A contractor should show the wall section and identify who is responsible for structural design when it is needed.
Price the kitchenette plumbing and electrical separately
A studio kitchenette may need a sink, faucet, base cabinet, counter, refrigerator, microwave, cooktop, or other appliances. The cost is driven less by the cabinet footprint than by the route to water, drain, venting, and electrical connections.
Ask the plumber to identify the proposed hot and cold water route, drain route, cleanouts, shutoffs, venting method, and any slab cutting. A short connection to an accessible nearby line is a different project from a drain that must cross the garage or connect through a distant house wall. The exact fixture and vent requirements vary by locally adopted plumbing rules and the approved design.
Electrical planning should cover general receptacles, countertop receptacles, lighting, smoke and carbon monoxide alarms where required, heating and cooling equipment, water heating, refrigerator, microwave, and any cooking appliance. Have an electrician verify service and panel capacity rather than assuming the existing garage circuit arrangement can serve a dwelling unit. Required circuit protection and outlet placement depend on the locally adopted electrical code and the project design.
If the project is being treated as a separate dwelling with substantial utility changes, confirm whether the utility, building department, or fire official requires additional metering, disconnects, shutoffs, or access. Requirements for an attached conversion differ from those for a detached ADU, so a detached-utility cost guide should not be applied automatically.
Include the complete mini-split HVAC scope
A ductless mini-split can be a practical option for a 400 sq ft studio when extending the home's ductwork would be difficult. The budget should include more than the indoor head and outdoor condenser.
- Heating and cooling load evaluation or sizing review.
- Indoor unit location and condensate management.
- Outdoor unit support, clearances, service access, and sound considerations.
- Refrigerant line routing, insulation, wall penetrations, and weather sealing.
- Electrical circuit, disconnect, and controls.
- Condensate disposal where the selected equipment requires it.
- Commissioning and confirmation that the equipment operates as intended.
Equipment sizing and installation requirements come from the manufacturer and the applicable local mechanical and electrical rules. An oversized system may cycle poorly and control humidity less effectively, while an undersized system may not meet heating or cooling needs. Get the model, capacity, efficiency, line-set limits, warranty conditions, and included electrical work in writing.
Treat egress windows as a design and safety item
A habitable studio may need an emergency escape and rescue opening, depending on the room arrangement and locally adopted residential code. Do not assume that any operable window qualifies. The applicable requirements can address clear opening dimensions, opening area, sill height, window operation, window wells, ladders or steps, drainage, and access from the interior.
Before ordering windows, ask the local building department which rules apply to the ADU and whether the existing garage wall can accommodate the required opening. The budget may include:
- Window unit and hardware.
- Structural framing or header work.
- Exterior flashing, weather barrier integration, siding, and trim.
- Interior jambs, casing, drywall repair, and paint.
- Window well excavation, drainage, cover, ladder, or steps if below grade.
- Safety glazing where required by location and configuration.
Verify the requirement with the authority having jurisdiction before finalizing the design. A model code provision may not be adopted unchanged in your city, and local amendments can affect the approved solution.
Organize bids and control overruns
Give each contractor the same drawings, finish schedule, appliance list, and written assumptions. Ask for a line-item proposal that separates labor, materials, allowances, permit-related services, taxes, and exclusions.
Use this bid checklist:
- Does the proposal include demolition and disposal?
- Does it include slab moisture testing, preparation, repairs, and the specified mitigation product?
- Does it show the complete garage-door infill wall, including exterior weatherproofing?
- Are windows, egress work, flashing, trim, and structural review included?
- Are plumbing connection distances and slab cuts described?
- Are the mini-split, electrical circuit, disconnect, condensate, and commissioning included?
- Are cabinets, counters, fixtures, appliances, flooring, paint, and trim included or listed as allowances?
- Who obtains permits, schedules inspections, and pays correction-related labor?
- What concealed conditions trigger a change order, and how will those changes be priced?
Prioritize moisture control, structural and envelope work, fire and life safety, utilities, and insulation before upgrading finishes. If the available budget is tight, simplify cabinet doors, counters, lighting fixtures, and flooring selections before eliminating required building work.
Garage conversion ADU FAQs
Is a 400 sq ft garage automatically suitable for an ADU?
No. The existing area may be large enough for a studio, but suitability depends on local zoning, minimum room or dwelling standards, ceiling height, natural light, ventilation, emergency escape, fire separation, insulation, plumbing, electrical service, and access requirements.
Can the existing garage slab remain?
Often it can remain if it is structurally sound, reasonably level, and suitable for the planned floor assembly, but it should be evaluated for moisture and water entry. A sealer alone may not correct active leaks, settlement, or excessive vapor transmission.
Does a garage conversion always need an egress window?
Not always in the same form, but emergency escape requirements may apply to sleeping rooms or dwelling spaces under the locally adopted code. Confirm the approved opening, sill, well, and access requirements with the local building department before selecting windows.
Is a mini-split the least expensive heating and cooling option?
It can reduce the need for new ductwork, but the complete cost depends on electrical capacity, refrigerant-line routing, condensate disposal, equipment sizing, and installation conditions. Compare a complete mini-split proposal with any permitted alternative rather than comparing equipment-only prices.
What causes the biggest garage conversion budget surprises?
Common causes include slab moisture or drainage problems, concealed framing damage, inadequate electrical capacity, difficult drain routing, required structural changes, exterior wall and fire-separation work, window wells, utility upgrades, and permit corrections. Early inspection and detailed allowances reduce, but do not eliminate, these risks.