There are a lot of things to consider. First are the appropriate regulations in your state regarding aboveground storage tanks (ASTs), for example, secondary containment volumes, release detection systems, etc. In Florida, ASTs less than 550 gallons do not require secondary containment. Your state most like also has a minimum cut off size. Second are the Federal requirements. For example, do the size of the tanks (note it is not just the size of the tanks but the total volume on site) require your client to prepare SPCC or SWPP plans? If so, then there are some very specific requirements and best management practices (i.e. mechanisms for discharging rainwater, and loading and unloading, etc.).
Regarding the design, if secondary containment is required for the tanks it will need to contain a minimum of 110% of the tank volume plus rainwater. Check the local regs for the storm event. For this reason, some tanks and containment areas are often covered. It kind of an economic balance between cost of constructing the roof and the secondary containment size. Lots of times, it is cheaper, safer and easier to install ASTs with built in secondary containment (see ConVaults, for an example, though I prefer metal tanks).
As for the pressure wash area, I would highly recommend adding a roof for this area and keep it separate from the ASTs. Once they pressure wash some nasty greasy part in the area, any water (i.e. rainwater) that comes into contact with the floor and sidewalls will need to be properly disposed of. Think of the cost of water disposal every time it rains even if they are not using the pressure washer. If the parts are small, there are pre-fab’ part washer units that contain all the rinse waters and work really well. If the parts are large consider using a suspended metal grate floor to stand on that way you can really slope the floor beneath to drain to a sump. Plus a grate floor will be safer to stand on. Most likely you will not need to install a pump in the sump, just provide access for vacuum truck which is how most of these kinds of water are collected. Also ask the client if they plan to use any solvents, you will need to check the chemical compatibility with you tanks and construction materials.
Finally MAKE SURE YOU SEAL THE FLOOR and SUMP!! I cannot tell you how many sites I have been to that floor and/or sump was not sealed and the soils and groundwater beneath the site were contaminated due to poor construction and/or design.
Sorry if I ramble on to much, I am new to the forum.