If the garage floor wasn't dug up and re-concreted then I doubt there is adequate insulation in the ground. The cold could be coming from the ground.

sounds to me like Mr Spyros is right in this case you can have all the insulation you like in the walls and roof space if there is none in the floor easy way to find out get a builder to use a curr bit but only get him to go 100 m 4inches down by then you will find any insulation and not go though the DPC ,or you can hire a drill with a core bit from a hire shop and do it your self if you dont find any insulation under the floor by then the chances are there is none

no ,they havent insulated the roof space at a guess ,common problem as this is quite a lot of expense and puts the true quotations to the top ,yhe homeowner being you picks the cheapest quote which obviously didnt include insulation to spec,which makes another question pop into my head ,BUILDING REGS,i suspect this has not been done to building control/regs or this problem would not arise as inspector would have made sure insulations used were up to date,PAY CHEAP PAY TWICE ,and in this case if no regs were cert were given on completion you will have a nightmare when it comes to re-sale,beware of cheap imitations.there are no bargains in building

Hi, this sound's strange, if as you say building control where involved, the new
bit should be warmer than the house. a silly question I know but has some
perhaps shut the radiator valve off for the summer months and not opened it
again, it usually turns out to be something silly like that.
Kind regards

Was this building inspector possibly just a mate of the builder? You'll probably find the insulation in the walls is jablite- the cheapest stuff on the market and meant to go in the slab. As said you'll get heat loss downward through the slab. Single glazing at windows and the wrong insulation in the loft. Insulation is expensive stuff - if the job was done cheaply then your answer lies in that fact.

This can be easily rectified, some form of internal insulation will make a big difference, the roof may also need insulating depending on what is there now, allow £1000 for insulation and plastering.

If the job was done under building control, i.e. insulation conformation, then it can only be inadequate heating supply.
Also make sure you do not turn radiator off and close door. I was called back to a conversion I had done and the owner had done just that to save money!!
I advised him to open the rad and open the doors and all was well.
Just check it has been signed off by your local council building control by calling your local council.

you need to check the insulation used if any both on ceilings and walls, also check the size of the radiator to see if its adequate a good plumber will check this for you

There isnt adequate insulation in the walls, along with possibly an inadequate heating arrangement.

Was this passed at every stage and signed off by building control? As stated by blacklion,sounds like insufficient insulation.If this is all above board then blacklions other suggestion is also probably correct.Bottom line is though that the conversion should feel like part of the house!