How far north do people build “real” in ground swimming pools in their backyard?
Do people have them in Maine? Canada?
It seems like a large expensive to build and maintain one for such a short summer season.
There’s an outdoor in-ground community swimming pool here. We’re about 20 miles north of the 45th N parallel.
Yes.
I’m in Winnipeg (49° 54’ N) and we have them here.
Yes, there are both public outdoor swimming pools and private (in peoples’ backyards) outdoor pools in Canada. Speaking from personal knowledge you’ll find them at the very least as far north as the 53rd parallel.
…but they’re a pain in the butt. Outdoor permanent pools often reduce the value of a house, because they are expensive and take so much care, but are only usable half the year tops, unless you live in BC. Many people don’t want to be bothered with them.
Mainer, here. I see lots of pools. Don’t be fooled into thinking Maine is a cold state. Temps in the 90’s during the summer are fairly common. Plenty of people swim(in lakes and pools) from memorial day to labor day.
Grew up in Castlegar, BC. Hot in the summer. We had an in-ground backyard pool.
One of the fondest memories of my life; and if I had a chance to buy a house in Victoria, BC with a pool, I would. (Not much chance with average house prices now $500,000, though.)
Edmonton has lots of backyard pools.
Useful, since they also convert to ice skating rinks the other eight months of the year. 
Interior of BC, here. It cleared a hundred degrees fahrenheit today. There are a lot of people who take advantage of the long, dry summer with an in-ground pool.
Mainer here too. We’ve got a pool as well, as do many around us. They also sell nifty pool heaters (solar, electric, and gas) to increase your season.
We typically use our pool from Memorial day until the beginning of October, depending on weather. But then again, we have a hot tub to jump in if the pool gets too chilly.
Pools really aren’t that much maintenance off season, especially once the pool freezes solid. Not much maintenance to do for a block of ice.
It’s expensive, but it can get very, very hot and humid here, so it sure is nice to have them.
Some people just like the luxury of having a pool. Having had one, I personally doubt whether it’s worth the bother, but boy, it’s nice to jump in one when it’s 33C and the humidity is 100%.
Don’t be fooled into thinking Maine is a warmish state. I know it gets hot there because I have witnessed it but you have to compare it to other states like Texas, Louisiana, and Florida during the same time. Other states get HOT for a long time and Maine is COLD by U.S. standards much of the year.
I wondered the same thing when I visited my now wife for the first time in the Boston area for the first time. They had a pool which I thought was odd but lots of people do in the Boston area. The problem is that these people think that a nice New England beach with 60 degree what is ideal for sunbathing and swimming. Most of the people I know with pools have them shaded and the water is chilly at best. I used to live in apartment complex that had a nice pool however and it was fine for 3.5 months of the year.
Remember that pools are for swimming and not just because it gets too hot to not be in the water. Do you know how cold the lakes are in northern climes? You can’t even wade into some to the knees without the legs cramping in the cold. Water quality sucks in most lakes too. Pools get chlorinated, and the bloodsuckers and alga are not present. Madison finally built a pool. The pool has to turn away persons, because the people don’t want to go to the contaminated beaches that are often closed.
45th parallel? Southern pansies! 
Almost all of the UK is above the 50th parallel, and there are plenty of outdoor pools here.
Here’s one near Manchester, about 53.5 degrees north.
A bit further north, here’s one in Edinburgh, Scotland, nearly 56 degrees north.
I grew up in South Dakota and we always had a public pool during the summer months. That was in the 1930’s and 1940’s.
That’s funny; I hadn’t realized that. Calgary has very few backyard pools, probably because of our more temperate summers, I guess. We do have outdoor pools, of course. Very few houses with A/C, too - it just isn’t practical for the two weeks of the year you need it. Well, older houses, anyway; I think they’re putting A/C into more of the new ones now.
The Scout camp a few miles away from me has a “solar heated pool”, as they call it. That’s in Ormskirk, Lancashire.
And we have them in Norway, so around 60 deg north.
Cool! Maybe I can make a new sig line:
Invisible Wombat: Writer. Bookseller. Montanan. Southern Pansy.
Well, maybe not.