We’re going to be extending our kitchen into the garage which currently only has a single skin brick wall. Do you think we can get away with just building a stud wall with celotex in between or will we need to build another brick layer with insulation between the two? Ideally would like it to be as thin as possible but still be enough to be well insulated.

Best is to go with a 40mm block with insulation, then insulted plasterboard to finish.
That way you have a decent substrate to fix anything to as well as it being toasty

Are there any piers in the single skin wall?

Look on the Trada website as they provide details (free) for timber constructions. My first pass is yes, insulated timber stud should be fine, but it’s the interfaces which may complicate matters.

Edit, if you are going to be fixing kitchen units to this wall, consider jumbo stud, effectively regular stud with a skin of plywood to provide additional strength and you can fix what you like, where you like.

Be careful with the detailing as external single skin masonry will get damp. Consider how you detail round openings and some kind of cavity tray at the dpc.

If we were using the 40mm block with insulation, what sort of thickness would we be adding to the inside of the wall? 40mm block + 75mm celotex, or would that not be thick enough?

Don’t forget that building control will want to know about this work – they will probably expect the floor to be insulated as well.

Following on from Rockhoppers post, garages are usually 100mm (min) below house floor level (for spillages believe it or not). Check if there is a difference.

There are no piers to work round. I had thought about the ply because of the kitchen units and to be honest I was rather hoping a stud wall would be enough, as I would imagine budget wise this would be the cheapest option? Saying that though, there’s no point doing it if we’re going to be spending a fortune on heating it and still sitting there shivering.

Our garage is quite a bit lower than the current kitchen, so we will definitely have to raise the floor with joists and insulate it. Still can’t believe the price of celotex. Whoever invented it must be absolutely minted!

We did the same, we blackjacked the internal wall to provide some damp proofing, 50mm air gap, 100mm celotex between studs and then 40mm insulated plasterboard. Met the England regs fine for U values.

For the floor we did 75mm celotex and then floor screen to match floor levels in house. We were lucky as floor on garage was 150mm less than kitchen.

We did ply sheet behind where units were going to be attached to provide a good fixing.

Building control were happy which was the main thin hand her indoors got the new kitchen she wanted.

Insulate then screed is a cheaper way to raise the floor as cdaimers has done. Joists sort of complicate matters in this instance.

Screed was so cheap and easy to do, mixed dry in mixer and now wish I had put in floor heating within the screed.

You should be able to do it , but like others there are a few details that will need working out.

For the floor simply use a floating floor