Buildings requiring a DEC  A DEC and advisory report are required for buildings with a total useful floor area over 250m2 that are occupied in whole or part by public authorities and frequently visited by the public. When DECs are required From 9 January 2013, if you are an occupier of a building requiring a DEC, you will need to display a DEC showing an operational rating in a prominent place clearly visible to the public. You will also need to have in your possession or control a valid advisory report.​Validity period of DECs Where the building has a total useful floor area of more than 1,000m², the DEC is valid for 12 months. The accompanying advisory report is valid for seven years.  Where the building has a total useful floor area of between 250m² and 1000m², the DEC and advisory report are valid for 10 years.