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We are moving into an old house very soon, the house has a seperate double garage with a pitched roof with 3 single rooms attached to it.
The construction is block wall and felt roof.
We want to make this a bit more habitable so we can use it all year round.
My idea was to block up the garage door, fit batterns to the inside of all the exterior walls at 450mm gaps and fill with insulation and then cover with plasterboard.
I am also going to plasterboard out the ceilings, create a loft hatch so I can fill with insulation.
Am either going to felt, shingle or fit lightweight steel tiles to the roof......depending on price.
I do not know what sort of damp proof the building has, what I am afraid of is after doing all this that later on I get the smell of mould and cannot see it as its behind the new plasterboard so have to rip all out looking for it.
What can I do to stop or remedy this.....is there something I can paint on the existing walls etc?
Other questions are:
What insulation to use for walls. Was looking at Kingspan but I think its out of my budget to be honest. What about Rockwall or Polystyrene?
What type of plasterboard for walls and ceilings. Foil backed etc?
Do I need a vapour barrier anywhere.
What would I need to do to the floor?

I really appreciate your help on this.

fix tanalised battens 2x2 vertically at 400centres paint the walls with buitumin paint 2 coats. Stick 50mm drytherm fiberglass on the wet buitumin affix polythene vapour barrier and plasterboard,keep the fixed plasterboard off the floor 25mm.
Thats the cheapest way I know how..

Have you ascertained if their is a effective damp proof course? Is it at least 6 inches above ground level, more is better. Are the walls dry?

If the walls are dry? Then the best idea is to use a parging coat of plaster to level the walls and to then stick plasterboard backed with polystyrene over. This comes in various thickness's, three inch thick polystyrene will ensure there is no possibility of condensation on the plasterboard and will give you a warm room.

Fitting three inch thick sheets of polystyrene across and below the joists finished with plasterboard will give you a warm effective ceiling.

Firestone make a rubber roof covering that comes in one piece made to measure, this will last for 50 years and will give you a quiet roof, steel will be very noisy.

Kingspan is a proprietary version of polystyrene, it has a designed life of 15 years or so, before it reverts back to the same level of insulation as polystyrene which is a lot cheaper and will give you the same level of insulation for the life of the building.

Fit a floating floor again three inches of polystyrene covered with T&G glued oriented strand board or finish of your choice.

You will end up with a warmer cheaper to heat building than your home.

Final point: Windows loose a great deal of heat, fit some modern double glazed units.