DEAP is the Dwelling Energy Assessment Procedure, the national calculation methodology used to produce a BER. It models fabric, ventilation, heating, hot water, lighting and renewables to estimate annual primary energy use and carbon emissions per square metre.
DEAP uses standardised occupancy and weather assumptions so that ratings are comparable between dwellings. It is not a prediction of your actual bills, which depend on how you live in the house.
Where the assessor cannot evidence a construction detail, DEAP applies a conservative default. Producing documentation for insulation, glazing and heating equipment often improves the rating without any physical work.