Establish the age of your building and any extensions that have been added.
Measure your property either externally of internally, depending on which is more practicable to gain the volume of heated space.
Go into your loft (if possible) and ascertain the amount of loft insulation you have.
Look at your boiler and hot water cylinder, any space heating devices and controls.
Check your fixed lighting fittings.
Look for any signs of cavity wall insulation.
Retrieve all data about any renewable energy systems fitted to the building.
Ascertain the type and age of any duouble glazing.