There are hundreds of informative articles about how to build and use a bat house. Check out such organizations as BCI (Bat Conservation International) for excellent guidance.

To learn if bats are on your property, begin sitting out at sundown, while it is still light. Bats find existing roosts in trees, under shingles, in carports, attics, gazebos, porch overhangs, etc. I've even heard of bats roosting under a patio umbrella!

They will be attracted to outdoor lights that attract insects....street lamps and porch lights, for example.

Even though we have a good amount of bats in our yard, I have yet to have a bat move into the bat house on eave of our shed. We'd love to attract more bats to our yard to eat mosquitoes!

I visited Austin, Texas quite a few years ago and they had a huge colony of several million bats that roasted under a cement highway bridge. I sat at an outdoor table at a nice restaurant at dusk with a drink and watched millions of bats stream out from under that bridge, following the flow of the river. It took about 30 minutes for most of them to vacate the roosting site. It was really fun to watch. My BIL is a vet who works with OBC (the Organization for Bat Conservation). There is a horrible infection that is wiping out huge populations of bats across the country. It causes a substance to collect on there faces that blocks their nose, making it hard for them to breath. They work so hard to breathe that they use up their stored fat too early and come out of hibernation before there are any insects available. Hopefully, some bats will be resistant and will find a way to restore populations with stronger immune systems.

Our house already had a bat house when we moved in, it's way up towards the roof. I'm sure there are bats because we see them in the evening and every so often we can hear noises up towards the roof, we assume it's them. I'm not so sure it helps with the mosquitos though, seems like those things are everywhere. We do have lots of dragon flys, frogs and birds too though so maybe our population is still less then what it would be otherwise.

I bought my dad a bat house probably more than 2 decades ago. It took him about 5 yrs to get the thing up. It took another....at least 3 yrs before bats found the house. Except for them migrating, the house is always full. He's got it on the side of his detached garage....and he's not really happy with it there.....bats do what bats do....and there's a pile of guano, a rather huge pile in his landscape rock....oops. I bought him another house, again about 5 or more years ago. It's still sitting. He keeps telling me he's going to put it up.....next year. Like my hubby and me.....ours has been sitting in the garage for at least 4 years :D

Don't count on bats (or purple martins) to help with your mosquito problem. Both species eat mosquitoes, but much prefer larger prey. Moths and night-flying beetles especially, in the case of bats. Mosquitoes make up a very small percentage of the diet (maybe one or two percent) of either species. They are just too small to be worth catching if anything else is available. And bat houses are "iffy" in my opinion. Bats seem to be much more selective in roost selection than birds are in selecting a nest site. Almost any kind of bird house you put up will eventually get a tenant of some sort, but I've seen bat houses stay vacant for years, even though they seem to be properly located and designed. But by all means put up a bat house anyhow, bats need all the help they can get. I don't know where you live, but here in the East bat populations have been reduced by 90 percent by white-nose disease, a fungal infection.