We do a lot of garage forecourts.
I had to let my insurers know we were working on them and it is noted on my policy documentation that I submit with RAMS.
We tend to work with the vehicles (and pressure washers) away from the pumps.
Works include canopies, soffits, all metalwork, car washes, kerbs, walls, floors and any adjoinig pavements.
There is an obvious issue with fire and ignition of fuel but a contained (vehicle) pressure washer positioned away from the pumps and possibly off the forecourt area completely is not going to cause a fire!!!
When you put your RAMS in make sure you let them know where the vehicles will be positioned and it should suffice.
Good luck
Rob