RobTristram

The latest (2023) U-value requirement for ground floors, whether in new houses or extensions, is 0.18. Apparently there is some complex way of calculating a reduction in the requirement based on the area and perimeter. You're allowed to use the whole house if you're designing an extension. The larger the floor area is compared with the perimeter, the less heat will escape, so the U-value of the floor can be higher than 0.18. The trouble is they don't tell you how to do the calculation, they just refer you to other documents. If anyone knows how to do it I would be grateful for their advice.