Let me know what you find out.
The assessor SHOULD be presently counting the pool house as living space, as it sounds like it has a roof, and 4 walls, like a guest house.
In 36+ years in this area, I haven't seen very many - if any at all - sun rooms considered living space, for assessment purposes. It probably depends on how substantial the structure is.
Such a project should not jeopardize your Prop 13 standing, although any added space ( That is now between the main house and your current pool room, for example.) might make for a slightly higher assessment from the tax man.
You may have a slight increase in your tax rate due to the addition, but it will be based on your prop 13 tax rate. I can't tell you how many transactions for purchases and refinances have fallen through because of the lack of permits. DON'T DO IT!!
Mortgage Loan Consultant
KINECTA FEDERAL CREDIT UNION
310 489-2549 Cell
To make it even more interesting, the pool room wall (where we would make the opening) joins a sun room that was enclosed in the 1960s and the previous owner never got permits to turn the sun room into interior space (or the permits were before the city microfiche archives). When we got some appraisals for an equity loan, appraisers were not completely sure whether or not to include the sun room as official interior space.