"A structure containing equipment used for a swimming pool." - https://en.wiktionary.org/wiki/poolhouse

"Pool houses are used for several different reasons by our customers: some use them as changing rooms, some use them to house the pumps and generator for their pool, some use them to relax in, and some use them as a combination of all the above with the help of a few partitions." - https://www.cranegardenbuildings.co.uk/articles/the-many-possible-uses-of-swimming-pool-houses