Granny flat designs and prices vary from about $35,000 for a basic conversion to more than $250,000 for a detached, fully equipped unit. Size, foundation work, utility connections, finishes, local labor rates, and permit requirements drive the final quote.
| Item | Low | Average | High | Notes |
|---|---|---|---|---|
| Garage or room conversion | $35,000 | $75,000 | $130,000 | Existing structure needs suitable access and utilities |
| Prefab detached unit | $90,000 | $150,000 | $230,000 | Site work and delivery can be separate |
| Custom detached granny flat | $130,000 | $210,000 | $350,000+ | Includes construction and typical interior finishes |
| Typical finished area | $150 per sq ft | $250 per sq ft | $400+ per sq ft | Small units often cost more per square foot |
What a 400- to 800-Square-Foot Granny Flat Costs
A finished detached granny flat commonly costs $120,000-$280,000 for 400-800 square feet, with an approximate midpoint of $190,000. A compact 400-square-foot plan may run $120,000-$190,000, while an 800-square-foot unit with a full kitchen and laundry may reach $200,000-$320,000. These estimates assume standard-quality finishes, ordinary access, and no major retaining wall or environmental remediation.
Conversions can cost less when the garage, basement, or existing outbuilding has a sound structure and usable utility lines. A conversion requiring a new bathroom, insulation, windows, electrical service, and plumbing can still cost $60,000-$140,000. Detached construction generally carries higher site and foundation costs than adapting existing space.
Assumptions: U.S. midmarket labor, standard materials, finished kitchen and bathroom, and normal site access; high-cost metro areas may exceed these ranges.
How Foundations, Utility Runs, and Finishes Shape the Quote
Contractors typically separate the estimate into site preparation, the shell, interior work, and service connections. The table gives broad planning ranges for a 600-square-foot detached unit; local bids may group these expenses differently.
| Quote component | Typical range | Common pricing basis | What changes the charge |
|---|---|---|---|
| Site work and foundation | $12,000-$35,000 | $20-$60 per sq ft | Soil, slope, excavation, slab or raised foundation |
| Framing, roof, and exterior | $35,000-$75,000 | $60-$125 per sq ft | Roof shape, siding, windows, structural design |
| Plumbing, electrical, HVAC | $20,000-$45,000 | $35-$75 per sq ft | Service-panel capacity and distance to connections |
| Interior labor and finishes | $30,000-$65,000 | $50-$110 per sq ft | Cabinetry, tile, flooring, fixtures, crew rates |
| Plans, permits, and inspections | $5,000-$20,000 | Per project | Jurisdiction, engineering, and review complexity |
Combined totals can vary because some bids include design, utility work, or contractor overhead in other line items. Ask each bidder to identify what is included before comparing total prices.
Which Size, Foundation, and Utility Specs Raise Prices
Size is only one driver. At under 500 square feet, a unit may still need a full bathroom, kitchen, heating system, and separate entry, so its cost per square foot can exceed that of a larger plan. Moving from 400 to 800 square feet may add $60,000-$130,000, depending on finish level and site conditions.
Foundation type and utility distance can shift a quote substantially. A level site with a simple slab may need $8,000-$18,000 of foundation work; sloped or poorly drained ground can push it above $30,000. Utility trenches of 20-30 feet are usually less costly than runs beyond 75 feet, which may require extra excavation, piping, wiring, or service upgrades. Confirm the planned floor area and connection distances during design.
Kitchen and bath specifications also matter: stock cabinets and basic fixtures can save thousands compared with custom cabinetry, premium tile, and high-end appliances. A 200-amp electrical service upgrade, when required, may add roughly $2,000-$6,000, subject to local conditions.
Ways to Keep a Granny Flat Project Within Budget
A straightforward rectangular footprint, standard roofline, and repeated window sizes reduce design and framing complexity. Keeping plumbing fixtures near one another can limit pipe runs; moving a bathroom or kitchen to the opposite end of the unit may add several thousand dollars. Choose the layout before requesting fixed-price bids, rather than changing it after construction starts.
Compare at least three detailed quotes using the same floor plan, finish schedule, site assumptions, and utility scope. Ask whether each price includes permit fees, appliance allowance, disposal, landscaping repair, and final inspections. A low bid that excludes trenching or site restoration may not be the lowest final cost.
Standard-grade finishes, a smaller footprint, and retaining usable existing structures can reduce spending without removing essential features. Keep a contingency of about 10%-15% for concealed conditions and price changes. Avoid paying for premium energy equipment or custom finishes unless they are required by code or valued for the intended use.
Prefab, Garage Conversion, or Custom Detached Design
Prefab units can shorten on-site construction, but the delivered price may exclude crane rental, foundation work, utility hookups, and permits. A basic prefab package may start around $60,000-$100,000 before site costs; a finished, installed unit often lands closer to $100,000-$230,000. Check the specification sheet for insulation, appliances, and finish inclusions.
A garage conversion often costs $35,000-$130,000 when the existing structure is sound, while custom detached construction offers more control over layout and accessibility at a higher total price. Conversion quotes should address moisture protection, ceiling height, fire separation, and windows or doors needed for legal occupancy. Compare complete, permitted project costs rather than advertised package prices.
How Location Changes Granny Flat Construction Prices
Labor and local requirements can move the same design well beyond national planning ranges. Major California, Northeast, and Pacific Northwest metros may price 15%-35% above a midmarket estimate, especially where labor is scarce or site access is tight. Many parts of the South and Midwest may come in 5%-15% below the national midpoint, though rural delivery and contractor availability can erase that difference.
Permits, impact fees, utility rules, and zoning vary by city and can add $2,000-$20,000 or more. Some jurisdictions allow streamlined accessory dwelling unit reviews; others require engineering, fire access, or sewer-capacity work. Request a written planning and fee estimate from the local building department before finalizing the design.
Three Sample Granny Flat Quotes by Project Scope
A 450-square-foot garage conversion with one bathroom, a compact kitchen, and standard finishes might total $75,000-$115,000. A sample budget could include 300-400 labor hours at $50-$85 per hour, plus framing, insulation, plumbing, electrical work, finishes, and permit fees.
A 600-square-foot prefab unit on a level lot might total $135,000-$205,000 installed. The package, delivery, foundation, utility connections, and interior completion determine where it falls in that range. A 750-square-foot custom detached unit with upgraded kitchen finishes and longer utility runs might cost $220,000-$330,000, with labor often taking 30%-45% of the quote.
These examples are planning cases, not fixed bids. Assumptions: ordinary soil, standard access, no major service replacement, and permits appropriate to the location. Written bids should state the floor area, finish allowances, labor scope, exclusions, and change-order rates.