Converting half a double attached garage

It will help if you read the other “garage conversion jobs”, particularly “Single Attached” regarding flat roofs, before ploughing through this article.

You know….just a bit of background we don’t particularly want to cover again.

So we now have a “2 car wide” attached garage, (ie it has 3 external walls). What you want to do is create a room in the half nearest the house and leave the rest as a garage.

So, just like before, let’s price for the most basic job you can do and build up from there.


The basic job (assuming a pitched roof)

A damp proof membrane, insulation and screed, to create a new floor on top of half the existing concrete one.

A correctly insulated, and fire protected, timber and plasterboard stud wall built right down the middle.

A correctly insulated, plasterboarded stud wall, directly behind the up and over door. (We’re assuming there are 2 doors, 1 for each side).

A vapour barrier, insulation and dry lining to convert the existing exterior rear brick wall.

An insulated, plasterboarded half ceiling, with ventilation above.

No plastering, just tape and fill the joints between the plasterboard and paint the room, (including the untouched internal house wall).

Fit a radiator, two electrical sockets, a central light and switch, skirting boards, architraving around the rear external door if you leave it in and probably a bit of boxing to hide the gas meter.

That will take two blokes on £150 and £100 per day respectively, 9 days, plus a sparks and a plumber for a day each …….

Labour………………….. £2650.00
Materials…………………£1400.00
Building control………. …£350


Basic Job Total …….£4400.00

The following prices are in addition to this basic job.

Maybe you also have to……..

Create a new flush internal door from the house…………£550.00

Actually dig out half of the old floor (because it’s too high possibly), and lay a new concrete “base slab” before undertaking the above, “basic job” floor work…………..£1000.00

Convert half the existing flat roof to a warm roof……..£750.00

This price takes into account the ceiling and insulation included in the “basic job” price.


Or, if you want to spend a bit more how about…….

Removing the garage door, putting in a new foundation and building a wall and window across the front, to “match” the rest of the house. This will be a bit of a must, or you will end up with a windowless room!

This price is minus the amount previously included in the basic job for masking the door. …………..£1300.00

Building an insulated cavity wall down the centre and the back wall rather than a timber stud wall.
This price includes laying a foundation (which would be done during the floor slab conversion).

We have also deducted from this price, the amount previously included in the basic job for building the stud wall…..£600.00

Plastering all the walls and ceiling……….£400.00

Putting in underfloor heating connected to your boiler…………..£550.00

This price is minus the amount we included in the basic job for fitting a radiator.