How much is a garage conversion depends on where you draw the line, and most quotes draw it earlier than most owners assume. A builder's price commonly ends at plastered walls and a screeded floor. What turns that into a room is flooring, decoration, joinery, the heating connection and the building control fee, and those five together are a substantial share of what you will actually spend.
Find the line the quote stops at
Ask directly: does this include flooring, decoration, skirting and architrave, internal doors, and the building control application. The answer varies by builder and neither answer is wrong. What is wrong is assuming, and then finding the room needs another trade and another fortnight before it is usable.
Budget the heating properly rather than optimistically
A radiator added to the end of an existing circuit in a room that used to be outside is often not enough, and sometimes not possible without rebalancing the system. Electric panel heating, underfloor in the new screed, or a small separate unit are all valid answers with different costs, and the decision is taken before the floor goes down.
Count the building control fee and the time it needs
An application, staged inspections and a completion certificate. It is not large as a share of the total and it is entirely non-optional, and the inspections have to fall before things are covered, which shapes the programme as much as the budget.
Do not forget where the contents go
A garage is usually storing something. Bikes, tools, a freezer, the Christmas decorations and the lawnmower need somewhere, and if that somewhere is a new garden store then it is part of this project's cost rather than a separate decision made in a hurry the week the work starts.
Other questions people ask
Is a garage conversion cheaper than an extension?
Considerably, and the gap is the structure: an extension builds walls, a foundation and a roof, and a conversion already has all three. What an extension gives that a conversion does not is additional floor area.
Can I do some of it myself?
Decoration, flooring and second fix joinery are where an owner can genuinely save without risk. The floor build up, the insulation, the electrics and anything structural are inspected work and are a poor place to learn.
What is the most commonly underestimated cost?
The floor, followed by the elevation. Both are invisible in a photograph of a finished room, and both are decided by the existing building rather than by anything you choose.