An energy assessment of your home will involve both an internal and external inspection of the property. For the purposes of obtaining an Energy Performance Certificate, they can only be undertaken by a fully accredited Domestic Energy Assessor (DEA) or Home Inspector (I will refer to both, as a DEA).

It is advisable - for your own peace-of-mind - to ask the DEA to bring along proof of accreditation when booking the appointment.

You could also try asking for their accreditation number over the phone and check its authenticity using the Government's central register of accredited Domestic Energy Assessors, before they even set-foot near your front door.

As part of a DEA's accreditation, a check on their Criminal Record is made with the Criminal Records Bureau (CRB) - It might be additionally comforting to know that the check (at the time of writing), is more comprehensive than those undertaken for such professions as, Gas Inspectors, for instance.

Energy Performance Survey

When the DEA arrives, he (or she) will need to inspect, and make note of, the following property details:

• The year of property construction
• Walls and roof construction
• Signs of wall-cavity insulation
• Energy-saving "green" devices (such as solar panels, for instance)
• Number of double glazed windows and when installed
• Number of open or closed fireplaces
• Make and model of boiler and the type of flue and fuel used
• Number of low energy light-bulbs
• Type of heating system used and controls (wall thermostat etc...) 
• Roof insulation and thickness.
• Measure the thickness of the hot water cylinder jacket or foam.
• Check for excessive window area in larger houses and take measurements of conservatories and extensions.
• Note the number of habitable rooms that are heated.
• Measure the internal or external areas of the home.
• Sketch and make notes relating to home and room layout

Hint  -it will make the process quicker and more accurate if you have to hand any paperwork confirming when works were carried out on your property such as new windows, extensions built  etc- the more recently these were carried out the better the energy rating for the property is likely to be.