Maximum L1B u-values for improved elements

When considering converting a building to create a residential dwelling it is important to note that the maximum permissible u-values for improved retained elements are:

Wall with Cavity insulation 0.55
Solid Wall 0.30
Floor 0.25
Pitched Roof (insulation at ceiling level) 0.16
Pitched Roof (insulation at rafter level) 0.18
Flat Roof 0.28

Basically, if you keep a wall, floor or roof from the existing building, the element must have insulation installed to reach the standards above. Unless this is achieved your building control officer is unlikely to sign off your development.