This is likely one of the first five achievements you'll pop. Building and upgrading huts requires resources. You can see how much in the game menu (
).
You can start building huts when your village population hits fifteen. I'd recommend waiting until you've bought Tensay back to the village first, by then you'll probably have more than enough resources.
At the village, Sayla and Tensay will be hanging around the spots where their huts are to be built - look for their icons on the mini map. Next to them will be a stone pedestal where you can build and upgrade their home.
NOTE - Your cave also counts towards the upgrading achievements. The pedestal is just inside on the right.