Four fire engines and around 25 firefighters were called to a fire involving two temporary office buildings on Bedfont Road in Stanwell.
Half of the ground floor and a large part of the first floor of the two storey buildings were alight.
Thirty staff evacuated the buildings before the Brigade arrived.
The Brigade was called at 14.56 and the incident was under control by 1624. Fire crews from Feltham, Hillingdon and Heathrow fire stations were in attendance.
The cause of the fire has been recorded as undetermined.