I'm putting an in-law suite in my basement. Any tips?
That is so nice of you – and very brave. I hope you all get along. If not, better locate it in the basement near the laundry room; they may never move in and, if they do, perhaps your mother-in-law will clean your husband's shirts.
In all seriousness, putting your in-laws in the basement is not ideal because they'll have to go up and down stairs and the light is limited. If this is your only option, make sure that there is natural light (this might require adding a window well) as well as pot lights and wall sconces (that they won't knock their heads on).
Build them an en suite bathroom with a shower that doesn't have a curb so that it's more easily accessed. And consider adding a small kitchen so they can have a light breakfast or a latte in the morning.
The best scenario, though, is an addition to the main floor with a separate entrance, giving you (and them) some much-needed privacy.