You found the flat, you like the area, and then you mention the dog. The landlord pauses. In Spain this is where a lot of rentals quietly die, and it is also where tenants get confused about a pet deposit that does not really exist in the law. Here is how it actually works.
Start with the words. Spanish law does not recognise a pet deposit as its own thing. There is the fianza, the legal deposit, and there are garantías adicionales, extra security the parties can agree on. When a landlord wants cover for a pet, they use those existing tools. Calling it a deposit for the pet is just shorthand, and it does not create a separate rulebook.
That matters, because tenants often believe the extra money buys them an automatic right to keep the animal, and landlords believe they can ask whatever a pet seems to warrant. Neither is quite true.
The real question is the clause. Ley 7/2023 on animal welfare did not hand tenants a right to keep pets in a home, and it did not outlaw no-pet clauses either. A clause that bans pets is generally valid if it is written into the contract and you sign it. If the contract says nothing about pets, they are not automatically forbidden, though a general rule from the comunidad or a building regulation can still bite. Assistance animals sit outside all of this and cannot be excluded by a clause.
For a normal vivienda lease the numbers are set, and a pet does not move them:
Since Ley 12/2023 this ceiling is explicit, and a landlord asking for four or five months because of the dog is simply outside the rules. You can say no, politely, and point at the cap. Some landlords then reach for a pet-specific insurance policy instead, which is cleaner for everyone.
It is not mysterious. It says how many animals and of what kind, that the tenant carries the pet civil liability cover, that the pet is not left to disturb neighbours, and that the tenant pays for professional cleaning and any treatment at the end if that is the deal. What it should not do is invent a penalty that has nothing to do with real damage.
Location changes the conversation. A city flat in Barcelona often comes with a comunidad that restricts pets in lifts, on the stairs or in shared areas, and neighbours who will absolutely complain. A villa in the Maresme with a fenced garden is a different proposition, and landlords there tend to care more about the pool, the fence height and who cleans up. If you have a large dog, the coast is usually the easier yes.
It is the paperwork and the wording that keep you out of trouble later, not a friendly verbal nod: