First off, sorry, I know this has been done before. I've just spent half an hour searching for advice on old threads, but I can't find what I'm looking for.
I get on very well with my ILs, they're lovely people. However, they are very much the head of the family, especially MiL, so no one on DHs side likes to challange them. We live close by, about 10 minutes away. I like having them over to visit, for a meal, or just a cup of tea. However, I am really not ok with them dropping in unannounced. We're busy most weekdays, so weekends are our time to relax, just me DH and DD. Sometimes I do just want us to sit down in our pyjamas, watching dvds! 
They turn up randomly at the weekend, usually because they've been out near by and it's a suitable time for them to pop round. They will ask what we've been up to, and if it's not busy enough/productive enough, or if the house is messy,they will make some kind of joke about it, which makes me bite my tongue to avoid making a sarcastic reply! They aren't nasty about it, it just puts me on edge.
DH and I both agree that it's not acceptable, and that we need to sort it out now before resentment builds. The question is, how? If I just say "Oh I'm sorry, we're busy" they will want to know with what, and will say they won't stay long! Not answering the door doesn't work, DD will toddle up to the door, with it's big windows, and they will know we are in. (or they will simply let themselves in with the spare key they said they didn't have!
) We need to bite the bullet and talk to them openly about it, but what the hell do I say? (Or rather, what should DH say?) I really don't want us to offend them with this.