Garage conversions are one of the most common construction projects in San Diego County right now, driven by California’s ADU rules and rising rents. The HVAC question almost always lands the same way: a single-zone ductless mini-split, $3,000 to $6,500 installed for a typical 400 to 600 square foot garage. Trying to extend the home’s central system into a former garage is usually a mistake because the existing ducts aren’t sized for the new load and the slab and walls weren’t built to be conditioned space. This guide walks the real options, real costs, Title 24, and the permit nuances that trip people up.
Why mini-splits are the default for garage conversions
Garages weren’t built as living spaces. They have uninsulated slabs, low ceilings (usually 7’6” to 8’), no return-air paths, and no existing ducts. Extending the home’s central AC means cutting new duct runs through finished walls or ceilings, derating an already-sized system, and often blowing through the SEER2 efficiency assumptions the original system was sized to. We see homeowners try this every month and end up with poor airflow in both the garage and the rest of the house.
A single-zone ductless mini-split solves all of that in one piece of equipment. One outdoor condenser, one indoor head, one set of insulated refrigerant lines through the wall. SEER2 ratings of 20 to 30, inverter-driven so it modulates instead of cycling, and quiet at 19 to 25 decibels indoors. For coastal garages in Pacific Beach, OB, Encinitas, and Carlsbad, the mild climate means a 9,000 or 12,000 BTU head usually handles the load. Inland conversions in Escondido, El Cajon, Santee, and Poway often need 12,000 to 18,000 BTU because summer surface heat through the slab and west walls is real.
The other reason mini-splits win: they’re heat pumps. One unit cools in summer and heats in winter, which matters for ADU rentals and Title 24 because California requires conditioned habitable space to be heated. A mini-split satisfies the heating requirement without adding a separate furnace, gas line, or wall heater.
Attached vs detached garage: the install changes
Attached garages share a wall with the house, which makes electrical and condenser placement easier. The outdoor unit typically sits on a slab or wall bracket on the side yard, refrigerant lines run through the shared wall, and the electrical comes off the main panel with a new dedicated 20 or 30 amp circuit. Permit-wise, City of San Diego treats this as a standard mechanical and electrical permit pair.
Detached garages are more work. The condenser placement is more flexible because you can put it on any wall, but you’ll likely need a sub-panel or at minimum a buried conduit run from the main panel. If the existing detached garage circuit was 15 or 20 amps for door openers and outlets, that won’t carry a heat pump load. Budget $1,500 to $3,500 for electrical work alone on a detached conversion. ADU detached conversions also need separate utility metering if you’re going by Title 24 part 6 for ADUs, though many San Diego County conversions stay sub-metered off the main house.
For both types, the slab matters. Garage slabs were poured uninsulated, often with no vapor barrier. Adding insulation on top (a sleeper system with rigid foam) raises the floor 2 to 4 inches, which compounds the ceiling-height problem. Without it, the floor stays cold in winter and the mini-split has to work harder. We see most San Diego garage conversions skip slab insulation to preserve headroom, then run a 12,000 BTU instead of a 9,000 BTU to compensate. That’s a fine tradeoff if you’re honest about it.
Real 2026 costs for San Diego garage HVAC
Pricing for a complete mini-split install in a converted garage, including equipment, refrigerant line set, mounting hardware, electrical, and permit:
- 9,000 BTU single-zone, attached garage, simple install: $3,000 to $4,500
- 12,000 BTU single-zone, attached garage: $3,500 to $5,500
- 12,000 BTU single-zone, detached garage with new electrical run: $4,500 to $6,500
- 18,000 BTU single-zone, inland garage with heavy west sun: $5,000 to $7,500
- Two-zone (rare for garage conversions, only for larger workshops or split-use): $7,500 to $11,000
Cost drivers that push to the high end: long refrigerant line runs over 25 feet (add $150 to $300 per additional 25 feet), main electrical panel upgrade if the panel is full ($2,000 to $4,500 separate), HOA architectural submission if you’re in a planned community ($150 to $400), and structural cuts through stucco for the line set on detached garages.
If you’re also doing the full ADU buildout (insulation, drywall, kitchen, bathroom, separate entry), the HVAC portion is a small line item. But it has to be coordinated with framing and electrical because refrigerant line sets need a clear path through the wall before drywall goes up.
Title 24, permits, and what San Diego inspectors look for
California Title 24 part 6 applies to any garage conversion that creates habitable space. The conditioning system has to meet minimum efficiency, the building envelope has to meet insulation requirements (R-13 minimum in walls, R-30 minimum in ceilings for our climate zone), and ventilation has to be addressed. A heat pump mini-split meets the conditioning side cleanly. The envelope side is the actual headache because garage walls and ceilings were rarely insulated to current code.
Permit-wise, City of San Diego requires:
- Building permit for the conversion (framing, walls, ceiling, floor work)
- Mechanical permit for the HVAC equipment
- Electrical permit for the new circuit, sub-panel work, or outlet additions
- Plumbing permit if you’re adding a bathroom or kitchen (most ADU conversions)
- Title 24 compliance documentation (HERS rater verification for the HVAC and envelope)
Inspectors specifically look at: line set insulation (closed-cell, not crushed), refrigerant charge documentation, condensate drain routing (to a code-approved drain or condensate pump, not just into the side yard), electrical disconnect within sight of the condenser, and the smoke and CO detectors required in any habitable space. The HVAC inspection is usually quick once the rest of the conversion passes framing and electrical rough-in. Plan for the HVAC install to happen after drywall is up so the indoor head can be mounted on finished wall.
County and incorporated cities (Carlsbad, Escondido, Oceanside, Chula Vista, El Cajon, La Mesa, Vista, San Marcos, Encinitas, Poway, Santee) all have similar requirements. Each has its own permit portal and fee schedule. Total permit cost for a garage-to-ADU conversion in San Diego County typically runs $2,500 to $6,500 depending on jurisdiction and whether you’re doing a full ADU or just a habitable rec room.
Decision framework
If you’re converting an attached garage to a bedroom, office, or rec room and the home’s central system is healthy: install a dedicated single-zone mini-split. Don’t extend the central system.
If you’re converting an attached garage to a full ADU with kitchen and bath: dedicated mini-split, heat pump version, sized for the actual conditioned space and load.
If you’re converting a detached garage to an ADU: dedicated mini-split, expect a sub-panel or full electrical run, plan for the higher cost.
If you’re converting to an unconditioned workshop or gym: you can sometimes get away with a portable AC and space heater, but if you’re spending real money on the conversion, a mini-split pays back in comfort within two seasons.
If the garage is going to stay a garage: don’t install HVAC. Garage door openings make conditioning a losing battle.
FAQ
Can I just extend my home’s central AC into the converted garage? Almost never the right call. Existing duct trunks weren’t sized for the additional load, and adding a run typically reduces airflow throughout the house. The HERS rater that has to sign off on the Title 24 portion will usually fail this approach. A dedicated mini-split is cheaper than rebuilding the central system to handle the new square footage.
What size mini-split do I need for a 400 square foot garage conversion? Coastal San Diego garages with good insulation: 9,000 BTU is usually enough. Inland or west-facing garages: 12,000 BTU. Detached garages with uninsulated slabs: 12,000 to 15,000 BTU. A proper Manual J load calc is required for Title 24 documentation anyway, so size off that, not a rule of thumb.
Do I need a permit for HVAC in a garage conversion? Yes. Anywhere in San Diego County, conditioning a garage as habitable space requires building, mechanical, and electrical permits. Skipping permits creates disclosure issues when you sell and can force expensive retrofits if a future inspection catches it.
Will a mini-split for a garage qualify for SDG&E or federal rebates? Yes, if the equipment is a heat pump and meets program requirements. The federal 25C tax credit is up to $2,000 for qualifying heat pump installs. TECH Clean California rebates stack on top, up to $3,100 for income-qualified households. SDG&E sometimes runs equipment-specific rebates as well. The contractor handles paperwork.
How long does a garage HVAC install take? A standalone mini-split install in an already-framed garage takes one to two days. If it’s part of a full ADU buildout, the HVAC rough-in is one day during framing and the final hookup is one day after drywall and trim, scheduled around the rest of the trades.
Can I use a window AC instead of a mini-split for a garage conversion? Title 24 will not accept a window unit as the primary conditioning system for habitable space because it doesn’t meet the efficiency, heating, or installation requirements. If the conversion is going to be permitted (which it has to be, legally), you need fixed equipment.
For a free quote on garage HVAC for your San Diego conversion or ADU, call us at (442) 777-6440. We coordinate with general contractors on conversions across the county and handle the Title 24 documentation. Related reading: mini-split HVAC for San Diego ADUs, ductless mini-split cost in San Diego, and HVAC new construction Title 24 in San Diego. For service in your area, see our mini-split installation page.