At what point do housecarls move into the home you build? Is it once you've built the beds?

I believe so.

Thanks. I wasn't sure if you had to tell them before they did so. I mean, I didn't want to finish the place and to find they were still living in the jarl's longhouse.

It's a nice big family. Two children, a cook, a bard, a housecarl and a dog. Oh and the pets that your kids bring home that die in one hit.

Far as I can remember, once you have some sort of a house built, they tell you they'll head there when you dismiss them as a follower, I'm fairly sure one playthrough I had Valdimar, only built the very basic house with no furniture, and he would hang around there if not with me.

First, which is obvious, become Thane of that Hold (you can buy the lot and not be Thane). Next as the previous post eludes to, you must speak to HC, otherwise they park themselves in the Jarl's place and not show up. Of course a house must be built as well.

There are times they do take off (Valdimar and Gregor especially) and either head to another HF home or it seems the Honning Brewery near Whiterun for some reason.

Hearthfire Housecarls move in once you build the entry way/small house (It does not matter if you do not build beds or not) and they will travel to the homestead.. By themselves.. With a chance of getting killed by a enemy tougher than them..