New dwellings must meet the maximum elemental U-values and the overall energy and carbon performance requirements set out in the edition of Technical Guidance Document L applying to the project. Because the figures have been tightened over successive editions, the correct values must be taken from the current TGD L rather than from memory or older publications.
Part L compliance for a new dwelling is not just a matter of hitting individual U-values. The regulations set an overall performance target through energy and carbon performance coefficients, backed up by maximum U-values for each element — walls, roofs, floors, windows and doors — so that no single part of the envelope is left substantially worse than the rest.
The exact figures depend on the edition of Technical Guidance Document L in force for your project, which is determined by the date of the application or commencement notice. Designers work from the published document and demonstrate compliance through a DEAP calculation carried out by a registered assessor.