I'd start with an Architect.

Check the local zoning ordinance. They may not allow and auxiliary building to be built without a main dwelling unit (or whatever terminology they use) on a lot. You can do that yourself or your architect can.

We're doing a new custom build that includes an inground pool and pool house. Our architect designed the pool house at the same time as our regular house. Our builder is building the pool house and doing the hardscaping while a pool company is building the pool. I'd start with an architect, especially if you want your pool house to flow with your main house.

And I disagree with @User. $250K is a reasonable amount for a nice pool and pool house, depending on where you live and what features you want. Our all brick pool house with half bath, kitchenette, outdoor shower and glass sliders didn't add more than $100K to our budget. The pool (with automatic pool cover) and hardscaping was another $100K.

Here is a pool house I designed years ago for an existing log cabin, I can not remember the construction cost. It set up on a hill and had beautiful views of Lake Michigan and an inland lake. The 16' x 40' pool is inside. If you can not build the pool house and pool on its own lot, you may be able to combine the lots. Your local architect will be able to help a lot. Have fun.

A stand-alone pool house on a separate lot would be treated like a new home with utility connections, permits & impact fees.

Budget depends on local costs today.

Thanks everyone for the input. Sounds like architect is the place to start. For some more context, I live in Grove, OK (northeast corner of state). $250k is less of a budget and more of an idea of how much I expect to have to spend for what we want. Any good experience with architects around my area or ideas on how to find one?

Look for projects under construction or recently completed that you like the style of the building and inquire who the architect is/was. Talk to the builder and home owner to get their experience. Talk to several architects to see their past design experience, how they would be to work with, and their fee structure.

If you are still working on this project, check with the San Juan pools distributor in your area. The closest may be Tulsa. A San Juan fiberglass pool may hold up better for you in your freeze/thaw conditions as well as unstable Oklahoma clay and earthquakes than a gunite pool would. Also familiarize yourself with the wide array of really beautiful pool cages that are available on the market today. You can get structures that are telescoping glass for varying seasons if you wanted to extend the swim season. I know it seems impossible with a quarter-million dollar budget that you might not get everything you want, but once you start adding in all the extras like water features, landscaping, privacy fencing, and a full kitchen/bath house, the size and scope of the project may change.