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A converted garage is a different building attached to your house
Uninsulated slab, a wall that was a door, almost no attic above it, and a heating system that was sized years before the space existed. That is why it is never comfortable, and why the usual answer makes the rest of the house worse.
Four things are true of nearly every one of them
The slab is uninsulated and often below the rest of the floor. A garage slab was poured to take a car, not to sit under a bed. It is usually thinner, usually has no perimeter insulation and frequently no vapour barrier, which is why these rooms are cold in January and clammy in August.
The wall where the door was is the weakest wall in the house. Filling a garage opening is straightforward building work and it is very often done with less insulation than the original walls, no proper air sealing at the header, and a window sized for light rather than for heat.
There is almost no attic above it. Garage roofs are usually shallower, with less depth for insulation and less ventilation, so the ceiling of a converted garage runs hotter than any other ceiling in the building.
And the heating system does not know the room exists. It was sized for the house as built. Whatever is being asked of it now, it was never designed to do.
What the space is, and how you were thinking of heating and cooling it
Nothing is sent anywhere and nothing is recorded. This is what we would ask you on the telephone.
The logic feels obvious: there is already a system, there are already ducts, so run a branch into the new room. The problem is that the system was sized for the house without that room, and a duct carries a fixed amount of air. A new register does not create airflow — it takes it from the rooms that were getting it before. People notice within a fortnight, usually in the bedroom furthest from the equipment.
Three measurements settle it, and all three can be taken in one visit: the calculated load of the new space on its own; the spare capacity of the existing equipment, which means what it is actually delivering rather than what is on its nameplate; and the static pressure and airflow the existing ductwork is running at. If the system is already at or over its rated static pressure — which, in this housing stock, it very often is — then there is no spare air to take and extending it is not an option at any price.
Where it genuinely does work: a small space, a system with real measured spare capacity, ductwork running comfortably within its limits, and a trunk close enough to branch from sensibly. It happens, and when it does it is the cheapest answer. We just will not assume it before measuring.
And one thing that is not ours to answer. In California a garage conversion is normally permitted building work, and permitted work carries requirements of its own about insulation, ventilation and how the space is heated. Ask your city or county building department, not us. We are a heating contractor and we will say so rather than guess on your behalf.
Treat it as its own zone, because it behaves like its own building
A bedroom has a narrower comfort range than any other room in a house, it is occupied when the building is at its coldest and its hottest, and it needs to be quiet. A converted garage bedroom fails on all three for structural reasons rather than equipment ones.
Before any equipment: find out what is actually above and below you. If the ceiling has less than a usable depth of insulation, adding it is the cheapest degree of comfort you will ever buy. If the slab is bare, a floor covering with some insulation value changes how the room feels in January far more than its cost suggests.
Then the equipment: a single ductless head, sized by calculation for that room. It is quiet, it runs at low output for long periods, which is exactly what a bedroom wants, and it heats as well as cools. Mounted high on the wall furthest from the bed, not above it.
What we would avoid: a wall heater as the only heat source, because it heats the air near itself and nothing else, and an oversized head, because at night it will short-cycle and you will hear every start.
Add up what is in the room before anybody sizes anything
An office or studio in a converted garage is the easiest of these to get right, because the load is knowable rather than guessable. Screens, computers, a printer, lights and a person all produce heat, and it is printed on the equipment in watts. A room with a lot of gear in it is a small heated building, and a room with a laptop is not.
The second question is when you use it. A room occupied from nine to five on a west-facing wall is at its worst exactly when you are in it. Shading that wall or that window is frequently worth more than a larger machine.
The equipment: a ductless head, sized for the calculated load rather than for the floor area. If the room is used for recording or video calls, say so before anything is chosen — sound level at low output varies a great deal between machines and it is the thing people regret.
And the ventilation: a sealed-up converted garage with a person working in it all day gets stuffy. That is an air change problem rather than a temperature one, and a cooling unit does not solve it.
This is a separate dwelling and it needs to be designed like one
Once there is a kitchen and a bathroom, the space has a cooking load, a moisture load, extract requirements and, usually, its own occupants with their own preferences and their own hours. Hanging it off the main house’s system is the wrong shape of answer even when it is physically possible.
What we would design: an independent system with its own controls — typically a ductless heat pump with one or two heads — so it can run on its own schedule and be metered or at least reasoned about separately.
And the ventilation is not optional here. A bathroom extract that actually moves air and is ducted to outside rather than into the roof space, and a kitchen extract sized for what is cooked. In a small, tight flat, moisture with nowhere to go becomes a problem quickly, and it shows up as condensation and mould rather than as discomfort.
What is not ours: whether the conversion is permitted, what the requirements are, and anything to do with the application. That is the building department’s and your builder’s. We will fit what the design needs and say plainly when a question belongs to somebody else.
Do the cheap things first — this is the one case where they usually win
A space still used as a garage, a gym, a workshop or a store is the easiest of these to make bearable without much equipment, because the standard is lower and the occupancy is short. It is also the one where people spend the most money unnecessarily.
In order: insulate the roof or ceiling if there is anything to insulate; insulate the garage door itself, which is usually a thin uninsulated panel facing the afternoon sun and is the single largest hole in the room; add ventilation so the hot air trapped at the top has somewhere to go; and get air moving with fans, which for a gym does far more for comfort than lowering the air temperature does.
Then, if it is still not enough, a ductless head sized for intermittent use rather than for holding a temperature all day. For a workshop where dust is produced, discuss filtration and extraction at the same time, because an indoor unit will collect whatever the room makes.
What we would not do: extend the house system into a space with a door that opens to outside. Whatever you put in goes straight out, and the rest of the house pays for it.
Better starting point — but the same question about the existing system still applies
A purpose-built addition usually has proper insulation, a proper floor and a roof designed to have a room under it, so it starts a long way ahead of a converted garage. What it shares is the important part: the existing heating system was sized before it existed.
If the addition is not built yet, this is the cheapest moment in the whole project. Get the load calculated against the drawings, decide then whether the existing system can carry it, and if ductwork is going to be run, get it designed rather than fitted into whatever space is left at the end. Retrofitting a duct route into a finished room costs several times what designing one costs.
If it is already built and uncomfortable, the sequence is the same as everywhere else on this page: measure the load of the space, measure what the existing system is actually delivering and what its ductwork is running at, and only then choose between extending, adding a head, or correcting what is already there.
About a converted bedroom specifically
Ask about sound level at low output, in decibels, from the person quoting. It is published for every machine and it is the thing people complain about six months later.
About an office or studio specifically
Add up the watts of everything in the room and bring the total to the visit. It takes ten minutes and it is the number that makes the sizing honest rather than assumed.
About a full flat specifically
Ask for the extract fans to be specified by airflow and ducted to outside, and ask where the condensate goes. Both are routinely left to the end of a conversion and both cause damage when they are.
About a gym, workshop or store specifically
Insulate the garage door before you buy any equipment. It is the largest uninsulated surface in the room, it faces the sun, and it is the cheapest thing on the list.
About a room addition specifically
If it is still on paper, get the heating and cooling designed at the same time as the framing. Everybody says this and almost nobody does it, and it is the difference between a duct route and a bulkhead.
If you were planning to extend the existing system
Read the note above before anything else on this page. In short: a new register does not create airflow, it takes it from somewhere else, and whether there is any to take is a measurement rather than an opinion. Ask for static pressure and airflow before you accept a price for a branch.
If you were thinking of a ductless head
Usually the right instinct in this housing stock. Two things to get right: have it sized by calculation rather than by room area, because an oversized head short-cycles and is noisy; and agree where the outdoor unit goes and where the line set runs before work starts, because both are permanent and both are argued about afterwards.
If you were thinking of a portable or window unit
A perfectly reasonable place to start, and the only sensible one if you rent. Make the exhaust hose actually reach outside and block the gap around it properly — a portable venting into a cracked window pulls hot air in from the rest of the building and people conclude, wrongly, that cooling does not work in their room. A window unit is more efficient than a portable for the same reason. Check your lease first if the building is not yours.
If you have not decided
Good. The decision should come out of two numbers rather than out of a preference: the calculated load of the space, and what the existing system is measurably delivering and running at. Get those and the answer usually chooses itself. Anybody willing to recommend a route before they have either number is guessing on your behalf.
Four things to establish before anybody quotes
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The calculated load of the space on its own
Room by room, against what the space is actually made of now — not the floor area, and not the size of the room next door.
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What the existing system is really delivering
Measured output and static pressure, against the equipment’s rated limits. This is the number that decides whether extending is possible at all.
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What the cheap building work would achieve
Ceiling insulation, door insulation, shading and air sealing, honestly assessed — including what they will not fix. We do not do that work, which is rather the point of asking us.
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Who owns the permit question
Your building department and your builder. Not your heating contractor, and not a page on the internet. Ask them before the equipment is ordered rather than after.