Some years ago building regulations only applied to new construction NOW THEY APPLY TO REPLACMENT AS WELL see pages 17, 18, thermal element. If you are unsure about any of this or someone tells you it is not needed, we sergest you contact your local building control office. To find which office you come under if you do not know, go to www.labc.uk.com there is a box where you can put in the postcode and it will tell you which building control office it comes under.
Put simply if the roof is over heated ,living area the regulations will apply, so a garage that is not normally heated, they would not apply. If they apply and the current level of insulation in the roof gives a U value of greater than o.35, which is the case with most roofs, then the insulation has to be improved to give a value of o.18 U. WARNING: local authorities do have the power to enforce these regulations. If non compliance is found at a later date when the property is sold for example it could be very costly to put things right at that point.