The short answer
Vaccination requirements with the timing, flea and worm treatment, and what to bring. Stated on the site, confirmed at booking.
That is the whole list. What follows is what each one has to say, because the difference between a requirement that works and one that does not is almost always the detail.
Vaccinations, with the timing
Naming the vaccination is not enough. The refusals at drop-off are timing refusals.
State which vaccinations you require, using the words that appear on an owner's card. State how many days before arrival the vaccination or booster must have been given. State what a cat that has never been vaccinated needs, and how long that course takes, so an owner booking three weeks out knows whether it is possible. State what happens when a booster has lapsed. State that the card must come with the cat or be uploaded at booking.
Add one sentence of reason: every cat in the building is protected by every other cat being protected. Owners follow a rule they understand and argue with one they do not.
Flea and worm treatment
Which treatments you accept, when they must have been given relative to arrival, and what you do if a cat turns up untreated. Most catteries treat on site and charge for it. Publishing that price turns a doorstep argument into a line item.
What to bring
Short list, plain words.
The vaccination card. Their own food for the whole stay. Any medication in original packaging with the vet's dispensing label, enough for the stay plus a couple of spare days. Their own vet's name and number. A familiar blanket or unwashed jumper if you allow them.
And what not to bring, which is just as useful: whatever you cannot wash or disinfect between cats. Say what you supply instead, so it reads as a standard rather than a refusal.
The rules that sit alongside these three
Not strictly requirements, but they belong on the same page because they are the same kind of surprise.
Arrival and collection windows, and what a late collection costs. Minimum stay, especially over peak dates. Whether cats from different households can share, and how sharing is priced. Neutering and age limits. Whether you take elderly or unwell cats and what you need from their vet. Your emergency vet arrangement and the consent it needs.
Stated is only half of it
A requirement on a page that nobody reads is a requirement nobody knows. Say it three times, at the three moments the owner is paying attention.
On the requirements page in full. In four lines beside the booking button, covering the vaccination timing and what to bring. In the confirmation email, with a link back to the page.
Confirmed at booking
The booking platform is where the requirement becomes a record. Revelation Pets, Gingr, PetExec and Kennel Connection all carry vaccination records with expiry dates on the pet profile, along with owner accounts and agreements signed in the flow.
Configure it so a first booking cannot complete without the vaccination detail and the consent, and switch on expiry reminders so a returning cat whose booster ran out in March gets an email in June rather than a refusal in August. That one setting removes most of the problem.
What not to do
Do not put requirements only in a terms PDF. Do not bury them in an accordion at the bottom of the FAQ. Do not write them differently on the page and in the booking form, because owners notice and then trust neither. Do not state a rule you do not actually enforce, and do not enforce one you have not stated.
Keep a date on it
Put a "last updated" line on the page. It signals to owners that the page is current, and it prompts you to check once a year that the page still matches what you enforce and what your insurer expects.
Where this sits
Requirements stated on the site and confirmed at booking is one of the six must-haves on a cattery website we build, sitting beside the routine page and the pen photography, and collecting records at first booking is standard across our pet business websites. What each package covers is on pricing. Write it once, say it three times, and nobody arrives with a taxi waiting and a card that is four days old.