A deep retrofit typically involves carrying out multiple energy efficient measures together, wall insulation, attic, replacing windows and doors, installing renewable energy technologies in the home. Where more than 25% of the surface envelope of a property undergoes renovation, the energy performance of the dwelling should be upgraded to achieve a cost optimal level where feasible. This typically requires wall insulation, attic insulation and the installation of a condensing boiler or heat pump or alternatively, the attainment of a B2 Building Energy Rating (BER).