I've just found this thread and its such a huge relief.
I really struggle to keep on top of my house. Not helped by the fact that it is in fact the size of a shoe-box and there are five of us and two dogs in it.
My biggest thing is that I never seem to get around to doing the floors. There's just so much stuff that needs shifting first and that combined with dealing with the every day disasters and chaos - particularly in the school holidays means I just never reach a point where its feasible. 
Visitors are strictly by appointment here as well, with the exception of my Mum who is a) used to it and b) very short sighted so doesn't notice the worst of the dirt. 
A month ago I did a major spring clean, cleaned everything from skirting boards, to picture rails - every bit of paintwork in the house, sorted every cupboard and cleaned every surface within an inch of its life. This was because the trustees of the house (it was inherited jointly by myself and my sister in trust) were coming for an inspection. Four weeks later and you'd think it had never been done. Its so frustrating I could scream!
Gentle otter, all three of mine do that - the boys currently have drawing and car designing on the kitchen table, a half built den in their bedroom and are in the process of using every single gogo they own (and there are hundreds) to design a massive picture on their bedroom floor - oh and ds2 has about 20 lego projects on the go as well and ds1 is reading two magazines and three paperbacks which are scattered throughout the house. 
I do have to admit that the gogo picture is pretty amazing though - they've sorted gogos into colour groups and then used them to do trees, the sea, the sun etc. I suspect ds2 is the one who 'designed' the picture and ds1 helped with the colours etc though as ds1 is unlikely to have been able to see gogos as anything but gogos. I am currently ignoring the chaos this little project has created because it has had them working together without fighting for an hour and a half now. 
Almost forgot dd (age 2) has a massive mega-blok tower, two jigsaws, a train track and a pile of musical instruments out on the living room floor as well.
Oh .... spoke too soon - ds's are now having a full on scrap.
It was nice when it lasted.