Make sure your extension is insulated to Building Regulations. You could Consider having the rest of your home to insulated as well. If your home has cavity walls, a layer of insulation can be blown into the cavity.
If you have solid walls (usually homes built before 1920), you can have either internal or external insulation added. Applying external insulation at the same time as your extension would ensure the exterior finish to the main building matches the extension, improving the aesthetic of your home.
External insulation will also make those cold rooms in your home warmer and, so long as sufficient ventilation is maintained, it can help alleviate many causes of damp and mould. Furthermore, solid wall insulation could save around £225 a year from the heating bills of a typical gas-fuelled semi-detached home. Savings could be significantly higher in detached properties, around £375 per year in a typical gas-fuelled detached home.