A granny flat is a small, self-contained living space on the same lot as a primary residence, built to host aging relatives, long-stay friends, or renters. The formal name is accessory dwelling unit, or ADU, and the term covers everything from a backyard cottage to an apartment over a garage. The U.S. Department of Housing and Urban Development defines an ADU as a habitable living unit added to, created within, or detached from a primary one-unit single-family dwelling, with its own kitchen, sleeping, and bathroom facilities. Most granny flats run between 400 and 1,200 square feet, and many are built over a flat plate floor system that keeps the structure low and the cost predictable.
The name varies by region and language. British English uses granny flat, while the same building shows up as a secondary suite, an in-law apartment, a casita, a tiny house, or a backyard cottage. Whatever the label, the unit shares the lot and usually the utilities with the main house. The appeal is financial and practical: the unit can house family, generate rental income, or do both at different times.
What Counts as a Granny Flat
The definition comes from how the building is used and zoned, not from its size. An ADU is a separate living unit with kitchen, sleeping, and bathroom facilities, belonging to the same real estate interest as the main house. Local rules add detail: most jurisdictions require a separate entrance, a minimum ceiling height, a parking space, and setbacks. Checking the local zoning text before designing anything avoids the most expensive mistake in ADU construction.
The HUD Definition and Local Rules
The HUD definition maps to the three ways a granny flat gets built. Detached units are new buildings in the yard. Attached units extend the main house. Interior conversions repurpose existing space in a basement, attic, or garage. Owner-occupancy rules vary: some cities require the owner to live on the property, and others have dropped the requirement entirely.
Structural Systems for Small Units
Most one-story granny flats sit on a concrete slab, and the floor system choice affects cost, speed, and flexibility. The flat slab construction system pours the slab and the supporting structure in one plane, which keeps formwork simple and build time short. Slab-on-grade construction avoids basement waterproofing and gives a low step-over height that suits aging residents, the original audience for the granny flat.
Typical Requirements Checklist
- Separate entrance from the main house
- Kitchen with a sink, cooktop, and refrigerator
- Full bathroom with a shower or tub
- Sleeping area that meets egress window rules
- Parking space where the zoning code requires one
- Setbacks and height limits from the local ordinance
Types of Granny Flats and How They Compare
Granny flats fall into three families: detached, attached, and interior conversion. Detached cottages sit in the yard and take the longest to build. Attached additions share a wall with the main house. Conversions reuse a basement, attic, or garage and usually cost the least because the shell already exists. The garage granny flat ideas circulating in design and construction circles show how far a conversion can go when the shell is sound.
Detached, Attached, and Interior
Detached units are the most flexible for layout and privacy but carry the highest cost because they need a full foundation, roof, and utility run. Attached units trade some privacy for lower cost, sharing a wall, a roof, or both. Interior conversions are the cheapest and fastest when the existing structure is sound, but they inherit the problems of the space, including low ceilings and damp basements.
Garage Conversions
Garages convert quickly because the walls, roof, and floor already exist. The work is mostly interior: insulation, drywall, a bathroom rough-in, a kitchenette, and a new door. Before starting, verify the slab thickness, the ceiling height, and whether the garage is attached to the main house, because attached garages bring fire-separation rules with them.
| Type | Typical size | Build time | Relative cost |
|---|---|---|---|
| Detached cottage | 400 to 1,200 sq ft | 4 to 9 months | Highest |
| Attached addition | 300 to 800 sq ft | 3 to 6 months | Mid |
| Garage conversion | 300 to 600 sq ft | 6 to 12 weeks | Lowest |
| Basement apartment | 400 to 900 sq ft | 2 to 4 months | Low, if dry |
Roofing and Waterproofing for Small Units
The roof is where small units fail first. Granny flats commonly use low-slope roofs to save cost and gain ceiling height, and the choice of roofing system determines how often the unit needs maintenance.
Waterproofing That Holds
On concrete roofs, the traditional brickbat coba waterproofing treatment for flat RCC roofs layers brick bats over a waterproofing membrane to add thermal mass and a walking surface in one system. Modern single-ply membranes achieve the same protection with less weight. Whichever system is used, a leak in a granny flat damages a rental income stream, not just a ceiling.
Membrane Options Compared
Modified bitumen, TPO, and EPDM are the three common membranes on small flat roofs. Modified bitumen is torch-applied and stands up to foot traffic. TPO is heat-welded, highly reflective, and easy to repair. EPDM is a rubber sheet that installs fast and handles cold climates well. All three need positive drainage and professional seams, because seams are where flat roofs leak first.
Roof Maintenance Schedule
- Inspect the roof twice a year, before and after the wet season
- Clear drains, scuppers, and gutters of leaves and debris
- Check flashing at walls, pipes, and skylight curbs
- Recoat or replace the membrane on the manufacturer’s schedule, typically every 10 to 20 years
Flat Roof Solutions for ADUs
The flat roof is the most common roof on a granny flat because it costs less, adds interior height, and connects simply to the main house. Small units often choose flat roof solutions because a low-slope plane is faster to frame and cover than a pitched roof. The trade-offs are real, and the design has to answer them up front.
Drainage and Slope
A flat roof is never truly flat. The code standard is a minimum slope of 1/4 inch per foot toward the drains, and the deck should be checked for low spots before the membrane goes down. Ponding water is the leading cause of membrane failure, so drains, scuppers, and overflow paths deserve the same attention as the membrane.
Insulation and Energy Performance
The roof also carries the insulation. A granny flat roof typically needs R-30 to R-49 in cold climates and R-25 to R-30 in mild ones. Rigid insulation above the deck keeps the membrane temperature stable and reduces condensation, while insulation between the joists is cheaper but leaves thermal bridges at every joist.
Light, Ventilation, and Interior Comfort
A small unit feels larger when it is bright, and the roof that saves money can also bring in the light. Skylights and clerestory windows turn a low-ceilinged unit into a pleasant one, provided the roof opening is flashed correctly.
Skylights and Daylight Openings
The flat roofs and skylights design and flashing guide covers how to cut a daylight opening without creating a leak. A skylight on a flat roof needs a curb that raises the glass above the drainage plane and flashing that laps over the membrane, never under it. Clerestory windows do the same work at the wall line and add cross-ventilation.
Ventilation and Egress
Every bedroom in a granny flat needs an egress window that meets the local size rules, and the bathroom needs mechanical ventilation that exhausts to the outside. Because the unit is small, the air changes quickly: a correctly sized bath fan clears humidity in minutes, and a range hood keeps cooking odors out of bedding and upholstery.
Costs, Permits, and Finishing the Unit
Costs vary by region, type, and finish level, but the ranges are consistent enough to plan with. Garage conversions run the lowest because they reuse the shell. Detached cottages cost the most because they are complete new buildings. Permit fees, impact fees, and utility connections can add thousands to the total, and the local ADU ordinance determines which apply.
What a Granny Flat Costs
| Type | Low end | High end |
|---|---|---|
| Garage conversion | $30,000 | $90,000 |
| Basement apartment | $40,000 | $110,000 |
| Attached addition | $80,000 | $200,000 |
| Detached cottage | $100,000 | $300,000+ |
Per square foot, finished ADU construction typically runs $150 to $400, with the spread driven by plumbing, finishes, and site conditions. A unit that rents for $1,200 to $2,500 a month in most metros recovers a six-figure cost in seven to ten years. Budgeting 10 to 15 percent for contingencies is standard practice, because site work and utility connections are the line items that surprise owners.
Permits and Financing
State laws have changed the picture in the last decade. California, Oregon, Washington, and other states now require many cities to allow ADUs by right, meaning the permit process is ministerial rather than discretionary. That shift removed the public-hearing hurdle that once stalled most projects. The remaining rules concern setbacks, height, parking, and utilities, and they vary block by block, so the local planning counter is the first stop.
Before You Build Checklist
- Confirm the lot allows an ADU and note the size, height, and setback limits
- Pull the permit before any site work, including utility trenches
- Verify water, sewer, and power capacity for a second unit
- Plan the parking space where the code requires one
- Add a 10 to 15 percent contingency line to the budget
Furnishing the Unit
The interior of a granny flat has to do more with less. Every piece of furniture should serve at least two purposes, and the wall space does the storage work that bulky furniture would otherwise claim. Built-in shelving, a bench with storage, and a folding dining table all keep the floor plan open.
Wall-mounted entertainment is part of the compact kit: a flat screen TV mounting setup frees the floor space a media console would take, and the bracket choice, from fixed to full-motion, changes how the room is used. A full-motion arm lets the screen face the bed or the sofa, turning one small room into two zones.
Lighting, storage, and quiet matter more in a small space. Warm LED lighting at 2700K keeps the unit feeling like a home rather than an office, and window treatments that block light for night-shift renters add insulation at the same time. A granny flat succeeds when the floor, roof, waterproofing, ventilation, and interior fit-out are planned as one project, from the first sketch to the last coat of paint.