Guest house and casita AC installation is the process of adding cooling to a detached structure on your property, whether that is a mother-in-law suite, a pool house converted into living space, or a casita that sits fifty feet from the main home. The goal is comfort that matches the main house, not a compromise. These structures have different needs than an interior room addition: separate electrical runs, longer duct paths if you tie into the main system, and often a building envelope that was never designed with air conditioning in mind.
- A dedicated standalone system usually beats extending ductwork more than 30-40 feet from the main house.
- Detached casitas need their own load calculation, not a square-footage guess borrowed from the main home.
- Palm Beach, Martin, and St. Lucie counties each handle permitting for detached structures a little differently in 2026.
- Panel capacity is the most overlooked step in ac installation guest house casita projects.
Why this matters for guest houses and casitas
A detached structure without proper cooling turns into the least-used room on the property. In South Florida's humidity, an uncooled casita also becomes a moisture problem fast. Furniture, drywall, and flooring absorb humidity when there is no system running to pull it out of the air, and that shows up as musty smells or soft spots in the walls within a season or two.
Many of these structures started as pool houses, cabanas, or garages before someone decided to live in them part-time. If yours followed that path, the AC installation guidance for pool houses and cabanas covers a lot of the same groundwork: sizing for a space that was not built to code for occupancy, and figuring out where the condenser can physically sit.
Snowbird owners add another wrinkle. If the casita sits empty from May through October, a standard thermostat setting will not protect it. Humidity keeps working even when nobody is home, and a system sized and programmed correctly is what keeps that guest house from smelling like a locker room when the owner walks back in for the winter.
How to plan AC installation for a guest house or casita
Decide between extending the main system and installing a standalone unit
Before anyone talks equipment, figure out whether your existing system has the capacity and physical access to serve the detached structure, or whether the casita needs its own dedicated system.
- Measure the distance from the main air handler to the casita. Anything past 30 to 40 feet of duct run starts losing meaningful airflow and temperature.
- Check whether your main system was sized only for the original house. Most were.
- Look at whether a trench or attic path even exists to run new ductwork to a detached building.
- Consider how the casita gets used. A guest suite occupied nightly needs different run-time than a hobby studio used a few hours a week.
- If the distance and access do not work, a standalone system with its own air handler and condenser dedicated to the casita is usually the cleaner long-term fix.
Run a real load calculation, not a square-footage guess
A lot of guest houses get sized off a rough "one ton per 400 square feet" rule, and that number ignores the details that actually matter in a casita.
- Count the windows and doors. Casitas built for natural light often have more glass per square foot than the main house.
- Note the insulation type in the walls and roof, especially if the structure was converted from a garage or pool house.
- Factor in ceiling height. Vaulted ceilings in a casita add real cooling load that a flat number misses.
- Account for occupancy pattern: a nightly-use guest suite runs differently than a space used a few afternoons a week.
- A proper AC sizing calculation applied to the casita specifically, not borrowed from the main house math, is the step that prevents an oversized system that short-cycles and never controls humidity.
Check the electrical panel before ordering equipment
Adding a second condenser and air handler means the property now needs to support two systems drawing power, and older Florida homes were not built with that in mind.
- Confirm the main panel has open breaker slots and enough amperage headroom for a second system.
- Check whether the casita needs its own subpanel, which is common when the structure sits any real distance from the main electrical service.
- Ask whether the existing panel is original to a home built before 2010, since many of those were sized for a single AC system only.
- Get a panel capacity check done before ordering equipment, not after it shows up on a truck.
- If a subpanel or panel upgrade is needed, that work has to be permitted and inspected separately from the AC installation itself.
Size the ductwork for a standalone system correctly
If a standalone system is the right call, the ductwork inside the casita needs its own design, not a scaled-down copy of the main house layout.
- Plan supply runs based on the actual room layout, including any lofted or open-plan areas.
- Size the return air path generously. Undersized returns are one of the most common airflow complaints in smaller structures.
- Route ductwork through conditioned space where possible to cut heat gain in the attic before the air ever reaches a vent.
- Seal every joint. A short duct run in a small structure makes any leak a bigger percentage loss than in a large home.
- Insulate ducts running through any unconditioned attic space, which in a Florida summer can hit well above 120 degrees.
Plan for permitting across county lines
A detached structure getting new mechanical work triggers a permit in every South Florida jurisdiction, and the process is not identical everywhere.
- Palm Beach County generally requires a mechanical permit plus inspection for any new AC installation on a detached structure.
- Martin County follows a similar process but with its own inspection scheduling windows, which matters if you are trying to have the casita ready by a specific date.
- St. Lucie County treats detached structures with living space differently than a simple utility building, so confirm the casita's occupancy classification before applying.
- A licensed contractor pulls these permits as part of the job, but ask to see the permit number once it is filed so you know it actually happened.
- Skipping the permit on a detached structure is a common shortcut that causes real problems at resale, when a home inspector flags unpermitted mechanical work.
Choose equipment built for a detached, exposed installation
A condenser sitting next to a casita is often more exposed to sun, salt air, and yard equipment than one tucked against the main house.
- Confirm the condenser location has enough clearance for airflow and service access on all sides.
- If the casita sits closer to the coast, look at AC brands with better resistance to salt air corrosion, since that outdoor unit will take more direct exposure than a condenser shielded by the main house.
- Match the equipment to a humid-climate system that actually pulls moisture out, not just one rated for temperature.
- Ask about a security cage or enclosure if the casita sits in a spot with less visibility from the main house.
- Get the SEER2 rating matched to the size of the space. Oversizing here wastes money without improving comfort.
Which installation option fits your casita
| Option | Best for | Key limitation |
|---|---|---|
| Extend ductwork from main system | Casitas within 30-40 feet of the main air handler with attic or crawlspace access | Airflow and temperature loss increase with every extra foot of duct run |
| Dedicated standalone system | Detached casitas used regularly, or structures too far from the main system | Requires its own condenser pad, electrical run, and permit |
| Convert an existing window unit to central air | Casitas currently running on a window unit that needs a permanent upgrade | Still requires new ductwork and equipment, so cost is closer to a full install than a patch |
The straightforward verdict: a standalone system dedicated to the casita is the right call for most detached structures in South Florida, because duct runs from the main house rarely stay efficient past the distance most casitas actually sit from it.
Get your casita sized correctly
A technician can run the load calculation and check your panel before any equipment is ordered.
Common mistakes with guest house and casita AC installation
- Sizing off the main house formula instead of the casita's actual load. A casita with more glass and less insulation than the main house needs its own number, not a percentage of the whole property's tonnage.
- Skipping the panel check until the equipment shows up. Finding out mid-install that the panel cannot support a second system delays the whole job and sometimes forces a change in installation date.
- Ignoring humidity control for part-time occupancy. A casita used only a few months a year, common with snowbird owners in Palm Beach and Martin counties, needs a system and thermostat setting built for that pattern, not the same setting as a full-time bedroom.
- Running ductwork through unconditioned attic space without insulation. In a small structure, the percentage loss from an uninsulated run through a hot attic is much higher than the same mistake in a large home.
- Treating the casita as exempt from permitting. Detached structures with living space still require a mechanical permit in Palm Beach, Martin, and St. Lucie counties, and skipping it creates problems when the home eventually sells.
FAQ
Can I run ductwork from my main house AC to a detached guest house?
Yes, if the guest house sits within roughly 30 to 40 feet of the main air handler and there is a physical path through an attic or crawlspace. Beyond that distance, airflow and temperature drop off enough that a standalone system usually performs better.
Does a casita need its own AC unit?
Most detached casitas do better with a dedicated system rather than an extension of the main house system, especially if the structure sits more than a short duct run away. A standalone unit also makes it easier to control humidity when the casita is used on a different schedule than the main home.
What size AC unit does a guest house need?
The right size depends on square footage, window area, insulation, and ceiling height, not a flat square-footage rule. A load calculation specific to the casita is the only reliable way to land on the correct tonnage for 2026 equipment.
Do I need a permit to install AC in a detached structure in Florida?
Yes. Palm Beach, Martin, and St. Lucie counties all require a mechanical permit and inspection for new AC work on a detached structure with living space. The process and inspection scheduling vary slightly by county.
Can my electrical panel handle a second AC system for a guest house?
It depends on the panel’s age and available amperage. Homes built before 2010 often have panels sized for a single system, and adding a second condenser and air handler may require a subpanel or panel upgrade before installation.
How do I keep a guest house from getting humid when nobody is staying there?
A system sized correctly for the space and set to run on a schedule, rather than shut off completely, keeps humidity from building up during vacant periods. This matters most for snowbird-owned casitas that sit empty for months at a time.
Is it cheaper to convert a window unit to central air in a casita?
Converting a window unit to central air still requires new ductwork, an air handler, and a condenser, so the cost is closer to a full installation than a simple swap. The upside is consistent temperature and humidity control across the whole structure instead of one room.
One last thing
The detail most owners miss is the condenser placement, not the equipment brand. A condenser tucked against a detached casita with poor airflow clearance on two sides will short-cycle and wear out years before a properly placed unit, regardless of the SEER2 rating on the label. Get the placement right in 2026 and the equipment choice becomes a much smaller decision than most homeowners think it is.
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