Tiny Homes, ADUs and Converted Garages

Why this matters

The small building behind the house is now one of the most common new dwellings in the country, and it arrives in three legal forms that look alike from the driveway: a permitted accessory dwelling unit, a tiny home on wheels that the law may treat as a vehicle, and a garage someone turned into a bedroom without telling anyone. Each one changes which code governs your work, whether you can pull a permit at all, and what the mechanical systems were sized for. The tech who reads the paperwork before the equipment avoids installing a gas appliance where it is prohibited, a heat pump three sizes too big, or a new circuit on top of a conversion that was never inspected.

The three things that look alike

An accessory dwelling unit (ADU) is a second, complete living unit on a lot with a primary home: detached (a backyard cottage), attached (an addition with its own entrance), or converted from existing space (a garage, a basement, an attic). It is a dwelling. It is built to the residential building code your jurisdiction has adopted, usually a version of the International Residential Code, with a permit, inspections and a certificate of occupancy or final sign-off. California also recognizes a junior ADU, a unit of up to 500 square feet carved out of the existing house.

A tiny home on wheels is built on a trailer chassis. Most jurisdictions treat it as a recreational vehicle or a park model, not a dwelling, which means it was built to a vehicle standard, typically NFPA 1192 for recreational vehicles or ANSI A119.5 for park model RVs, and its electrical system falls under NEC Article 551 (recreational vehicles) or 552 (park trailers) rather than the dwelling articles. Some states and cities now allow them as permanent residences under specific rules, and many still do not allow anyone to live in one full time in a backyard. A tiny house on a foundation is a dwelling, and the IRC has a tiny-house appendix for units of 400 square feet or less (Appendix Q in the 2018 edition, relettered AQ later), which only applies where your jurisdiction has adopted it.

A garage conversion is either a permitted ADU or a junior ADU, or it is an unpermitted room. The physical clues are the same either way. The paperwork is not.

A manufactured home built to the federal HUD code is a fourth thing, with its own rules. See related: Manufactured Homes: What Is Different for Every Trade.

The state layer: ADU law, and why California sets the pace

ADU rules are mostly state law on top of local zoning, and the state layer has grown fast. California's is the broadest, found in its Government Code and reorganized into its own chapter by 2024 legislation. Among its terms, which apply to California ADUs only: a local agency must act on a complete ADU permit application within 60 days, the review is ministerial (checked against objective standards, not a discretionary hearing), and units under 750 square feet are exempt from impact fees. Several other states have since required cities to allow ADUs: Oregon and Washington are two examples, and more join every legislative session. Other states leave the whole question to local zoning, where an ADU may be allowed by right, allowed only with a hearing, or banned.

What that means for your work: in a strong-ADU state, a homeowner who wants a heat pump in a new backyard unit can get a permit quickly and predictably, so there is no reason to skip it. In a state with no ADU law, the same unit may not be legal at all, and the permit question may be "can this building be a dwelling," which is far above your scope. These laws move every year; treat anything here as current as of 2026 and check your state housing agency and your city's planning department before quoting.

Small mechanical systems and the oversizing trap

Small, well-insulated units have small loads, and the equipment market starts above many of them. The most common failure in ADU and tiny-home mechanical work is oversizing: a heat pump or furnace big enough to satisfy the space in minutes, which short-cycles, never runs long enough to pull humidity out in summer, and wears its compressor. The smallest common ductless mini-split heads are rated around 6,000 to 9,000 BTU per hour of cooling, which is more than some 300-square-foot units need at design conditions. Run a proper load calculation (ACCA Manual J is the residential standard) for the unit's actual insulation, glazing and orientation, and choose variable-capacity equipment whose minimum output sits near the calculated load.

Water heating. Small units often get a point-of-use or small tankless heater. Check that the electric supply can carry an electric tankless unit, whose demand is high for a small building, and that a gas tankless unit has a proper vent path.

Electrical. A detached ADU is usually fed from the main house by a feeder to its own subpanel, and the main house's service has to carry both. A load calculation under NEC Article 220 for the combined dwellings comes before adding a heat pump, a range or an EV charger to either. A tiny home on wheels connects through a shore-power style inlet, commonly rated 30 or 50 amps, and its pedestal is site wiring under a different set of rules than the trailer's interior.

Plumbing. The sewer lateral may tie into the main house's line, so a backup in the ADU can be a main-house drain problem. A tiny home on wheels may use a holding tank, an RV-style connection or a composting toilet, and a plumber should know which before promising a fix.

The converted garage: what changed when the cars left

A garage built as a garage was designed for different hazards than a bedroom, and a conversion inherits them unless someone dealt with each:

• The slab usually has no vapor barrier and no insulation, so the floor is cold and can wick moisture into new flooring.
• The water heater or furnace that lived in the garage was raised off the floor because garages hold gasoline vapors, and it drew combustion air from a large, leaky space. Once the room is a bedroom, the rules for appliances in sleeping rooms apply instead: the International Fuel Gas Code, section 303.3, where adopted, prohibits fuel-burning appliances in bedrooms and bathrooms unless they are direct-vent or meet the code's specific exceptions.
• The garage door opening was infilled, sometimes with an uninsulated wall and sometimes with the door simply left in place behind drywall.
• The circuits were sized for a garage: often one or two general-purpose circuits now feeding a bedroom's worth of loads, possibly with a space heater.
• The fire separation between garage and house was a required barrier; once both sides are living space, the door between them and any openings matter differently, and a permitted conversion addresses them.

Worked example: a mini-split request in a converted garage

A homeowner asks for a ductless mini-split in a 400-square-foot converted two-car garage now used as a bedroom and office.

First, the paperwork. The tech asks whether the conversion was permitted. The homeowner is not sure. The county's online permit search shows a permit for "garage conversion to ADU" with a final inspection two years ago. That answers the big question: the room is a legal dwelling space, and the mini-split can go under an ordinary mechanical permit.

What would have changed it: no permit on record. Then the mini-split itself is still a permittable piece of work, but the inspector who comes for it may see an unpermitted bedroom. The shop does not hide that from the customer; it tells them before pulling a permit that the inspection could raise the conversion, and lets them decide, in writing, whether to legalize the conversion first or proceed knowing that. The shop does not do unpermitted work to avoid the question.

Second, the appliances already in the room. A 40-gallon atmospheric-vent gas water heater still sits on its 18-inch stand in the corner, behind a closet door with a louvered panel. The room is now a bedroom. Under IFGC 303.3 as this jurisdiction adopts it, an atmospheric appliance in a sleeping room is not allowed unless it sits in a dedicated enclosure with a solid, weather-stripped, self-closing door and all combustion air drawn from outdoors. A louvered closet door is the opposite of that. The permitted conversion should have caught it; either the closet was altered after final, or it was missed. The tech does not install around it. He notes it, explains to the homeowner that it is a carbon monoxide and code issue, recommends a direct-vent or heat pump water heater, and records that the homeowner was told.

Third, the load. A Manual J for the room, with its now-insulated walls and two windows, comes out at about 7,000 BTU per hour cooling. The smallest single-zone head the shop stocks is rated 9,000 nominal, but its variable-capacity compressor turns down to roughly a third of that, about 3,000, so it runs long, steady cycles at this load. A fixed-speed 12,000 unit, which the homeowner's neighbor suggested, would be over 1.7 times the load at full output with no turn-down, and would short-cycle.

Fourth, the circuit. The garage was fed by one 20-amp, 120-volt circuit, now carrying lights, outlets and a computer. The mini-split needs its own 240-volt circuit. The main panel has two open spaces and the load calculation for the house shows room, so a new dedicated circuit goes in. Had the house been at capacity, the job would have paused for a load-management or service-upgrade conversation rather than squeezing in a circuit.

The failure the sequence prevented: a correct mini-split installation completed in a bedroom that still held an atmospheric gas water heater, with the shop's name on the last permit for the room.

Permits, and when to walk away

Pull permits on ADU and tiny-home work exactly as you would on the main house. The temptation to skip them is higher in a small, out-of-sight building and the consequence is the same: unpermitted work is hard to insure, hard to sell, and lands on the last contractor whose name is on it. A tiny home on wheels raises a separate question your license may not answer: in some states, work inside an RV falls under a different licensing or inspection regime than building trades. Confirm with your state licensing board before taking recurring work on them.

Walk away, or route to the right professional, when the question stops being about your trade: whether a building may legally be occupied, whether a structure meets fire separation, or whether a trailer can be permanently connected to utilities in that city. Those belong to the building department and, for anything contested, the owner's own attorney.

References

• See related: Manufactured Homes: What Is Different for Every Trade
• See related: Reading a House's Age from Clues on Site
• International Fuel Gas Code, section 303.3, in the edition your jurisdiction has adopted, on appliances in sleeping rooms
• ACCA Manual J for residential load calculation; NEC Articles 220 and 551 in your adopted edition
• California Department of Housing and Community Development ADU guidance, and your own state housing agency