The bakery chain’s branch at Sandy Lodge Service Station, in Sandy Lane, between Oxhey and Northwood, has ceased operation to allow for the refit to take place.
Works are scheduled to finish on Wednesday, April 15 and it will reopen the following day.
The Asda Express store at the service station will remain in operation throughout and motorists can still fill up on the forecourt.
There is no shortage of Greggs locations in the Watford area and the nearest alternative stores are in Station Road, Carpenders Park, and High Street, Rickmansworth.
The branches are around a five minute and ten-minute drive from the service station respectively.
Wenzels also has a bakery branch near the end of Sandy Lane, in Maxwell Road, Northwood.