In this guide
3. Floor insulation
The installation of insulation in your floor must meet the minimum energy efficiency values set out in the Approved Documents.
However, if such an upgrade is not technically or functionally feasible, the element should be upgraded to the best standard which can be achieved within a simple payback of no greater than 15 years.
Renovation of more than 25 per cent of a solid or suspended floor involving the replacement of screed or a timber floor deck would have to meet the standards required by the building regulations Approved Documents.
Read more about changes to thermal elements (on the 'External Walls' page).