The short answer
Somewhere legal and large enough to park, access to water and power where your van needs them, an adult at home for the appointment, and a dog that has been out to the toilet and not just fed. Say all of it on the website and repeat it in the booking confirmation.
That is the answer. The reason it needs saying twice is that owners have no way of guessing it.
Why owners get it wrong
They are not being careless. They have never had a van at their house before, and the mental model most of them have is a plumber: someone who turns up, parks somewhere, and gets on with it.
Nobody who has not had a mobile groom knows the vehicle needs a space the length of a small van with the doors able to open, or that a two hour groom uses more water than a kitchen tap fills in a minute, or that a generator is not always the answer on a terraced street at nine in the morning. Left unsaid, all of it is a surprise on the doorstep, and the surprise costs you the whole slot.
Parking
Be specific about what you need rather than saying "space for the van". Say roughly how much room, that the back doors need to open, and that you need to be able to stand beside it.
Then name the situations that catch people out: permit-only streets where they may need a visitor permit, restricted hours, gated estates and barrier codes, narrow lanes, low branches or height barriers, and shared driveways where a neighbour's car appears at the wrong moment. Ask them to keep the space clear rather than to find one when you arrive.
Say what happens if there is nowhere to park. A stated policy, applied evenly, is far easier than an argument on the pavement.
Water
Say what your van carries and what you need from them. Many vans carry a tank that covers a full day and need nothing; some need to top up between dogs; some work from the outside tap directly.
Whichever it is, write it down. If you need a tap, say you need one you can reach from where the van will stand and that a hose connector helps. If you carry your own water, say that too, because that sentence removes a worry the owner did not know they should have.
Power
Same approach. If you run everything off leisure batteries or an inverter and need nothing, say so plainly, because it is a selling point in a flat or a terrace. If you need mains, say whether an outside socket is required or a lead can be run from inside, roughly how much you draw, and whether the cable will cross a path so a mat or a cover is sensible.
If you use a generator, say so honestly and say what you do about noise, because a neighbour complaint on the first visit ends the second one.
The other things worth stating
An adult at home for the appointment, or a clear agreement about what happens if not. The dog let out for the toilet beforehand and not fed in the hour before, especially for a large or nervous dog. The dog on a lead at handover rather than loose in an open hallway. Other dogs and children kept back at the doors, because the van is a working space with clippers and scissors in it. Somewhere to hand back the dog if it is raining.
Where to put it
Three places, in this order.
A short requirements page, linked from the header and from the booking flow, written as a list a person can scan in fifteen seconds. Two photographs alongside it, of the tank or hookup and the power set-up, so it reads as an explanation rather than a set of demands.
Then the same questions inside the booking itself, in MoeGo, Savvy Pet Spa, Fresha or Booksy, asked as intake fields before a slot is confirmed. Parking, tap, socket, distance from the door, and whether someone will be in.
Then the confirmation and the reminder, both carrying a short version of the list with the arrival window. Nobody reads a page they saw three weeks ago. Everybody reads the text that lands the night before.
The tone
Write it as helping them get a good groom, not as a list of conditions. "So the van can get set up quickly and your dog is not kept waiting" does the same job as a warning and reads like somebody who has done this before.
Where this sits
A requirements page, the same questions inside the booking, and the list repeated in the confirmation are part of every mobile grooming website we build, and setting expectations before arrival runs through our other pet business websites. What each package covers is on pricing. Say it three times and you stop losing mornings.