I can totally relate to your feelings. I could not be happy living in those conditions no matter who I was living with!
How about putting together a schedule for the renovations with strict deadlines and a financial plan for getting it all done. You have to really express how important this is so he knows he can't just say okay,okay and then blow it off for another five years or so. You have to let him know this is making you think about leaving and that you don't want to leave so you need his help to correct this situation.
Yeah, it sounds like your place needs major work, and when a place is already in sad shape it's pretty easy to be 'a pig' because you figure the rest of the house looks like hell, one more thing not done isn't gonna matter much.
The other option you have is to just GET ANOTHER PLACE. Unless you really cannot afford it, it sounds like almost anything would be better; Maybe in an apartment or condo or something that you can get a fresh start in, it would be easier to say, hey, pick your socks up, or help me dust or whatever. You wouldn't be dealing with huge major things that need to be dealt with to clean up and are easy to put off because they seem so daunting.
Just because his parents own the house doesn't mean you HAVE to stay there - it sounds like another case of taking the path of least resistance. If you can afford to get a halfway decent apartment to live in, then you could work in whatever time you have to fix up the house without having to actually live there at the same time, getting in your own way. And still go home to a nice clean residency at the end of the day.

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