Here’s the trap with garage conversions.
The walls are up. The roof’s on. The floor’s already concrete.
So it looks like you’re halfway done before you’ve even started. You’re not.
That “almost finished” look is exactly why the price swings from £10k to £40k. And the difference is almost never the paint, the plaster, or the new window. It’s the stuff you can’t see — under the floor, behind the walls, and up in the roof.
Let me show you where the money hides, so you find it on paper instead of halfway through the build.
First, the reframe: a garage isn’t a room. It’s a box for a car.
Your house is built to keep people warm and dry. Your garage is built to keep a car out of the rain. Different brief entirely.
That means an unheated, often single-skin box, with a floor and roof that were never meant to hold heat or block damp.
Turn it into a bedroom or office and you’re not “finishing” it — you’re upgrading it to house standards: warmth, damp control, and (often) structure.
Check those three early and you control the budget. Find them late and the budget controls you.
The 5 things that actually decide your price
Not the flooring. Not the spotlights. These:
1. The slab — “it’s already concrete” is the trap
Most garage slabs aren’t insulated. Some weren’t even laid like a proper house floor — thin concrete, and none of the continuous damp-proof membrane that NHBC standards expect under a real ground floor.
To make it warm and dry, you add insulation and a new build-up. That lifts your finished floor 100–150mm.
Sounds tiny. It isn’t. Now the door won’t open, the step’s in the wrong place, and the ceiling feels low. Cheap to design. Expensive to discover.
2. The walls — one skin of brick won’t keep you warm
Your house has cavity walls. Your garage often has a single 100mm leaf — sometimes less. (Building Regs Approved Document A lets a garage wall be as thin as 90mm. Fine for a car. Useless for a bedroom.)
To fix it, you line the inside with insulation. That eats 50–100mm off every wall, complicates your windows, and quietly shrinks the room you were picturing.
3. The roof — the quiet deal-breaker
Garage roofs are built to shed rain, not hold heat. Often flat, shallow, or with nowhere to put insulation.
Force insulation in without getting the detailing right and you don’t just get a cold room — you get condensation and mould. The classic “looked fine from below” problem.
4. Pulling out the garage door — that’s structural, not cosmetic
Taking the door out feels like the easy bit. It isn’t.
That big opening is usually holding weight. Fill it in and you’ve built a new wall — and new walls need foundations.
NHBC is clear: foundations on a garage have to cope with differential settlement. Realistically that’s 450mm deep minimum in normal ground, and 750mm or more in shrinkable clay or near trees. Miss it and you’re digging up the front of the house mid-build.
5. Knocking through to the house — where the real money lives
Open it up to the house and you’re cutting into a load-bearing wall. That means a beam — steel or concrete — sized to the load, sitting on padstones, with a clear path for that weight to travel all the way down to solid ground.
Approved Document A is the rulebook engineers work to: floor spans up to 6m, beam bearings of 100–150mm, and openings that can run to around 5m wide when they’re properly supported.
“We’ll just make it open-plan later” is exactly how a tidy conversion turns into a structural project.
Not sure which of these apply to your garage? WhatsApp us your questions and we’ll be able to assist.
It has to hit thermal standards now
The second it’s a heated room, the floor, walls and roof have to meet the energy targets in current Building Regs (Part L).
Ballpark, that means pushing walls and floors toward roughly 0.18–0.30 W/m²K, and tighter again on the roof — but Part L moves, so treat those as a guide and confirm the live figures with your designer.
For builders and architects, the real point: insulation thickness and junction detailing have to be designed in from the start, because they drive your ceiling heights and wall build-ups.
The cheap conversion vs the expensive one
Two garages can look identical and cost £30k apart. Here’s what makes one cheap:
- A cooperative floor — you can insulate and damp-proof it without creating weird levels.
- Walls that line out cleanly — insulation goes on without killing the room size.
- A roof you can actually insulate — warm, dry, no condensation.
- No heroic structural surgery — or the openings were planned properly from day one.
Tick those and you’re in £10k territory. Miss them and you’re climbing toward £40k.
Why your “two quotes” aren’t the same quote
The visible stuff — plaster, sockets, windows, heating, flooring — is the predictable bit. Every builder prices it about the same.
The price explodes on the invisible stuff: the slab, the wall build-up, the roof, and the structure around new openings.
That’s the whole £10k-vs-£40k gap. And it stays invisible in a quote summary right up until someone opens things up.
The one move that saves you the most: survey first
Before you commit to a layout, order windows, or sign off a price — get a structural survey.
Done early, it tells you the expensive stuff up front:
- Is the slab going to need serious work?
- Can the walls be lined cleanly, or is it a bigger job?
- Can the roof actually take insulation?
- Are the openings you want simple — or structural?
It costs a few hundred quid to change a drawing. It costs thousands to redesign a structure once you’ve started. That’s the trade.
What an engineer actually looks for (it’s not cracks)
A structural engineer walking your garage isn’t hunting for cracks. They’re tracing risk and cost:
- How the garage was built compared to the house
- what changes the moment it becomes a heated room
- where the loads go once you cut new openings
- where insulation and damp control are easy — and where they’re a fight
In one sentence: the difference between “yeah, we can make this work” and “this is going to snowball.”
Bottom line
Garage conversions can be brilliant value. But only when the structure and the fabric are on your side.
If the slab’s not insulated, the walls aren’t cavity, or the roof can’t take insulation, your tidy little conversion can quietly become a rebuild. Add structural changes — pulling the door out, knocking through to the house — and the surprises stack up.
So do the one thing that keeps you on the right side of that £10k–£40k line: find out what’s hidden before you commit.
Thinking about converting your garage? WhatsApp us your questions and we’ll be able to assist.
