My experience over the yeara with, certainly with concrete sectional garages is that they are a disaster; leaks everywhere, roofs are prone to damage, impossible to seal and make weather tight over the long term.
So when we wanted a workshop and gin palace in our garden we went with a custom wood 'shed' that an ex-neighbour who used to do this kind of thing as a business made. I did all the lining and stuck a cedar shingle roof on it about 7 years ago. It's slightly L shaped 7m wide and 4m at it's deepest on a concrete base, mega insulated and all lined. The woods all treated with a fire retarder so has a 60min fire rating, it's got alarms and extinguishers.
There's power, it's got wired network and local wireless.
Does it leak? Nope
Does it stay warm? Yep
Does it get cold? Nope
It's essentially a pretty simple thing to build, you take a load of wood, make a load of frames and screw them whatever you have as a base. Make a few fink or scissor trusses to hang a roof onto, if you are really flash a clerestory truss roof which I kind of wish we'd done on ours, might go that way when the roof needs replacing in a few years.
Nail a skin of T&G over the top, slap on a roof and then get the services in.
As long as it's not too tall or has too big a foot print it's permissive development, though that does bear checking.
I'd honestly say if you go to a company that sells or make 'garden rooms' you are going to be royally shafted.
The cheapest option by far is to get a base laid or a strip foundation done by a small building company then find a local chippy because the whole thing is essentially based on panels so anyone that can make up internal partition walls can do the main construction. Any good sparky can lay in wiring, link it to the house supply and certify it.
Avoid anything 'garden sheddy' on skinny 2x2 frames, they're cheaper but that's because they're flimsy, you want chunky 4x2 for the wall panels and roof trusses. Same for the skin fence panel grade stuff is cheap but its not going to last 5 minutes.
The next cheapest is to get something prebuilt that you can have thrown up on the base, there are loads of systems which essentially are a bit like log cabins, the wood just slots together.
In all of these the materials are the expensive part, our 'shed' has literally a bit over a ton of wood in it, some double glazed doors, windows and roof lights and a huge pile of cedar shingles hundreds of the bloody things
The price of timber has gone up considerably of late. Sourcing enough decent salvaged timber, something like a big truck load of old roof trusses would work but take ages to collect plus very few demolitions these days do anything but smash everything up and landfill it.
At the time ours ended up costing about half the price of a more commercial 'shed' for over double the size. Still came out at a few grand though.