A mini split heat pump is the standard way to heat and cool an ADU in Los Angeles. A studio or one-bedroom up to about 550 square feet usually needs one 9,000 to 12,000 BTU unit, and a larger ADU with separate rooms needs a multi-zone system. For permit applications filed since January 1, 2026, California’s Energy Code also calls for a heat pump in a newly built ADU and for new heating in a garage conversion.
Key takeaways
- Size by floor area first, then by layout. A closed bedroom door matters more than 50 square feet.
- The 2025 Energy Code treats a new detached ADU as new construction and a garage conversion as an addition.
- New or replacement space heating that serves an ADU addition must be a heat pump.
- 9,000 and 12,000 BTU systems are available in 115 volts, which can save panel space.
- Rebate eligibility depends on whether the property is in LADWP or Southern California Edison territory.
What size mini split does an ADU need?
Use floor area for a first estimate. The capacities below come from the ENERGY STAR sizing chart, which assumes an 8-foot ceiling, matched to the sizes mini splits are made in.
| ADU floor area | ENERGY STAR chart | Typical mini split setup |
|---|---|---|
| 300 to 400 sq ft studio | 8,000 to 9,000 BTU | One 9,000 BTU unit |
| 400 to 550 sq ft | 10,000 to 12,000 BTU | One 12,000 BTU unit |
| 550 to 700 sq ft | 14,000 BTU | One 15,000 BTU unit, or two zones if there is a closed bedroom |
| 700 to 1,000 sq ft | 18,000 BTU | Two or three zones |
| 1,000 to 1,200 sq ft | 21,000 BTU | Three zones |
ENERGY STAR adds 10% for a very sunny space and subtracts 10% for a heavily shaded one. A load calculation for each room sets the final size.
One indoor unit or several?
One indoor unit conditions the room it is in. It does not push air through a closed door. That makes layout the deciding factor.
- Studio or open plan: one indoor unit on the longest wall.
- One bedroom with a door: a unit in the living area and a second in the bedroom, both on one multi-zone outdoor unit.
- Two bedrooms: usually three zones, one per bedroom and one for the living area.
Do you need a mini split in every room? covers the layouts where you can use fewer. For how single-zone and multi-zone systems differ, see multi-zone vs single-zone mini splits.
What California’s 2025 Energy Code requires for an ADU
The code treats an ADU differently depending on how it is built. The 2025 Energy Code took effect January 1, 2026 and applies to permit applications filed on or after that date. This summary follows the California Energy Commission’s ADU fact sheet.
| Type of ADU | How the code treats it | Heating requirement |
|---|---|---|
| New detached ADU | Newly constructed building | The space conditioning system must be a heat pump in all climate zones, or the building must comply through the performance path |
| New attached ADU | Addition | New or replacement space heating serving the addition must be a heat pump |
| Unconditioned garage converted to an ADU | Addition | New or replacement space heating serving the addition must be a heat pump |
| Existing conditioned space converted to an ADU | Alteration | Requirements depend on which components are altered |
Two ventilation points catch owners out. A new ADU needs whole-dwelling mechanical ventilation whatever its size. An ADU addition of 1,000 square feet or less is exempt from that, but its kitchen range hood and bathroom exhaust still have to meet the code. Separately, the California Residential Code requires a habitable room to have permanent heating that can hold 68°F, so a portable heater does not pass on a permitted ADU.
Check with your designer: your plans and the building department that issues the permit decide what applies to your project. Use this section to know what to ask.
Power and metering
An ADU mini split needs its own dedicated circuit. A 9,000 or 12,000 BTU single-zone system is available in a 115-volt model that takes one breaker space. Multi-zone systems need a 230-volt circuit.
If the ADU has its own meter or subpanel, feeding the system from it puts heating and cooling on the tenant’s bill. That is one reason an ADU usually gets its own system instead of an extra indoor unit off the main house.
Working with your builder
The easiest time to plan the system is before the walls are closed. General contractors send us the plans, and we walk the site with them before we bid. Three decisions are worth making at that stage:
- Where the outdoor unit sits, so it clears setbacks and windows. See where a mini split outdoor unit should be installed.
- Where the line set runs, so it can go inside the wall instead of in a cover on the outside.
- Where the condensate drains.
Rebates
Heat pump rebates in Los Angeles are run by the utilities, and LADWP and Southern California Edison have separate programs. Check current terms on our LADWP Electrification Program and SCE heat pump rebate pages. For pricing, see how much it costs to install a mini split.
Frequently asked questions
What is the best heating and cooling system for an ADU?
For most ADUs it is a ductless mini split heat pump. It heats and cools from one system, needs no ductwork and satisfies the Energy Code’s heat pump requirement for new ADUs and ADU additions.
What size mini split do I need for a 500 square foot ADU?
The ENERGY STAR chart puts 450 to 550 square feet at 12,000 BTU. If the ADU has a bedroom with a door, plan on two smaller indoor units instead of one.
Does an ADU need heat by code in California?
Yes. The California Residential Code requires habitable rooms to have permanent heating that can maintain 68°F. Under the 2025 Energy Code, new heating in a new ADU or an ADU addition must be a heat pump.
Can one mini split serve both the main house and the ADU?
It can be done with a multi-zone system when the line lengths allow it. A separate system is usually the better choice, because it keeps the ADU running if the main system fails and makes separate billing possible.
Can a mini split be installed in a garage conversion?
Yes. A garage conversion is one of the most common places for one. See what size mini split a garage needs.
Planning an ADU?
Send us the plans or book a free consultation. Dr. Ductless is a Mitsubishi Electric Elite Diamond Contractor and has served Southern California since 2001.
Call (213) 916-0005 Contact usSources
- California Energy Commission, 2025 Energy Code: Accessory Dwelling Units
- ENERGY STAR, Room air conditioner sizing chart
- California Residential Code, Section R303.10, heating facilities for habitable rooms
Buying a property that already has an ADU? Have its heating and cooling checked before close of escrow. Our sister company handles HVAC inspections in Los Angeles for exactly that situation.
