Technical Guidance Document L sets a maximum permitted air permeability for new dwellings, measured by a pressure test and expressed in cubic metres per hour per square metre of envelope area at 50 pascals. The applicable limit should be read from the edition of TGD L that applies to the project.
Part L sets a backstop air permeability figure that a new dwelling must not exceed, and the tested result also feeds into the DEAP calculation used to demonstrate overall energy and carbon compliance. A better result than the backstop improves the calculated performance and the BER.
Because the target has tightened over successive editions of the guidance, the applicable limit depends on the version in force for your project. Your designer or assigned certifier will confirm which figure applies and will usually set a design target below it to allow a margin.