It is also a must in an apartment and if not a must then atleast an asset (also for a landlord)! Have you ever thought why Finnish people love to sweat naked in a steamy hot room? But before answering I must ask you that have you ever tried a real Finnish sauna? Not the ones that Swedish people call sauna because that's not it at all. Swedish version is merely a warm room, they don't blow of steam like finn's do. It is time of relaxation and purification and it is really a passion of mine.
After hard exercise or sweaty renovation it is the best sensation to get a refreshing shower and hop on lauteille of the hot steamy sauna and open a cold beer. Also every swimming stadium in Finland has public saunas and after sauna a dip in the pool and couple of strokes back and forth, I recommend.
There are couple of things that Finnish landlords also appreciates in their apartments. Not just just the dishwasher or washing machine. Own sauna and a balcony in the apartment are huge game changers. Even though sauna boosts up the electrical bill and is quite often just some sort of a storage room. And Finland is not that warm so balcony is pretty much out of season during winter times, so pretty long time of the year, but that too is nice to have. But those are said to add more than 10-15 thousand euros to the apartments value than compared without them.
I have already renovated one sauna and I liked it. It does have it ups and downs wanting to do everything yourself. But then there is no one else to blame and I always like learning new stuff and skills (and spare the labor costs). I have now 2/4 apartments with sauna and for me it is a must. I am also considering adding a sauna to one which is soon going to be completely bathroom renovated. It merely depends on the regulations and costs.
I will keep you posted and I dare you to test a Finnish sauna, test wooden heated if possible it is the right way. 80 degrees +/- 10 degrees, that should keep you warm.