But traditional summer cottages and villas have been either intentionally or still built wood burning stoves unless three phase power is easily available not bring cost up too much because remote location and long distance to grid. We have about half a million summer cottages in Finland. Which almost all have saunas and I would guess that perhaps 5% would have electric saunas as most summer cottages are built quite long time ago and off grid.
There are fancy (luxury) summer cottages where there is not one but either two or even three saunas built or moved there. All different types of course if having many. One electric inside for convenience.
Traditional (continuous) wood burning sauna, "jatkuvalämmitteinen" in Finnish, right next to lake because that type is consider to give better 'löyly' (steam in sauna) than you get from electric stove and thus preferred by many.
Third if some have is usually oldest type, the smoke-sauna. Which is really nice to have if you can afford keeping and have patience to make use of it few times a year. It takes lot of time and bit of knowledge too to warm it up which can take up to 6-8 hours, before it's ready to start bathing there. This was most common type about hundred years ago in country side.
Fourth type is or mostly was between smoke-sauna and continuously burning stove sauna. Its stove burns wood during heating, but then during bathing it's just releasing heat accumulated during heating. This type name in Finnish is "kertalämmitteinen kiuas" ie. onceheated-stove. And was most common in towns and cities before continuously warming stove was invented and became popular about 60 years ago.
I go sauna four times a week, once evening where I live and three times a week early in morning when I go swimming to (county owned) swimming baths.
e: typos, and clearer expressions.
[EDIT: I should have single phase, which is more conventional ]
But if you buy an old summer cottage further away from permanent living areas it may well be that a) you don't even have grid there or b) if you have it's single phase and three phase upgrade would be too expensive because you are being billed building cost for that work all in front.
Using that single phase for sauna stove needs then so much that it's not allowed by code or if you would be able to convince some electrician do some kind fo switching other devices off when stove is on most perhaps do not like to pursue that and choose wood stove their sauna instead. That's known working solution and remote location it's also a secondary heat source incase grid were down due some storm fallen trees on wires which mess cleaning takes several days etc.
2. If you have 230V single phase, not sure why you couldn't run a sauna from that, unless there is some other heavy load to be run concurrently.