Maximum wall U-values for dwellings are set in Technical Guidance Document L, and separate values apply to new dwellings and to walls in existing buildings undergoing material alterations or extensions. The applicable figure should be read from the current edition of TGD L for the relevant work type.
Wall U-values are governed by Part L, with different maximum values for new dwellings and for works to existing dwellings such as extensions or material alterations. Achieving the required figure depends on the construction type: a filled cavity wall, an externally insulated wall, an insulated timber frame and an ICF wall all reach the target in different ways.
In practice the U-value is calculated for the whole build-up including bridging by wall ties, studs or mortar joints, so the insulation thickness alone does not determine the result. The designer's calculation, not a rule of thumb, should be used to specify the wall.