I don’t know how my recent Barcelona experience will apply to San Sebastian, but for what it’s worth:
The apartments can be referenced with several agencies. It’s up to the owner. There may be exclusivity if the agency also manages the rental. Agencies here will only show an apartment that is listed with them. They work for the landlord and have an agreement that allows them to collect a fee from the tenant (= one month or 10% of a year), unless the landlord is an entity, in which case the landlord must pay (but expect you at what cost to add to the rent).
There are people/companies that help with the apartment search and charge the tenant for the service. If they know what they’re doing, they could help avoid mistakes like renting a place in an area that will be very noisy when the drunken crowds come out of the bars at 3am and that sort of thing. Agency fees will still need to be paid since these people are not agents, they are just helping you find and secure a place. There are owners who do not use agencies. Someone who assists you may be better able to connect with these landlords and also weed out scams (which you should be wary of). Sometimes the service is hourly, sometimes fixed.
Some agencies want you to sign a reservation contract, which is a very one-sided contract where you pay them an amount equal to their fee (which would then be converted into their fee) to stop listing the apartment pending the conclusion of a rental agreement. . This doesn’t stop other agencies from continuing their efforts to rent out the apartment, so you really don’t get anything in return. I was presented with one (1) in which I promised to either enter into the lease or waive payment without first seeing the contract; and (2) with a multi-week lease entry deadline with no guarantee that the landlord would sign, essentially keeping me on the hook during that time. I told the agent to forget it.
A word of advice, when you find a place you like, visit the area in the early hours to see what it looks like afterwards. Unless it’s well insulated with double-glazed windows, has no quiet neighbors, etc., you could be miserable.
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