I have to ask. With a 4000 ft home how much is your heating bill usually....ours is a couple hundred per month and we have a lot less space than that. Seems like that would be close to your normal bill with or without them. Just had to ask.
I would say you love your daughter very much to put up with this for this long.
It is easy for us to say do this or do that. When it comes to the child you carried in your body it is hard to be harsh with them.
I think you have already started to accept there will be changes coming. You would not have posted this question without that feeling in your heart.
You live in 4000 sq. feet of home. You have enough space for them. I imagine you don't feel like you are on top of them all the time anyway, I may be wrong though. We lived with hubby's mom and dad for a few months and it was hard. She would not stay out of our room. I would come home from work and she would have gone through my drawers and straightened them or rearranged them. She would have gathered the laundry and washed and dried it. She would have come in and cleaned the bathroom, etc....I was livid and felt invaded and I was totally pissed. Looking back now I know she was doing her best to try and help me be a working wife, to make my life easier, and here I made her life hell while I lived in her home.
She died a few years ago and I miss her every day. She was the best person and totally a role model I can look up to and try to emulate.
The house out there is 4 bedrooms, 3 bathrooms, 2 living areas, dining room, office, and they closed in the deck to make a workshop for my FIL. So it is a huge space.
We had only been married a couple of years so we were always "in the mood". The master suite is off the living room where the huge screen TV is so if we wanted to do the deed we had to wait until everyone was in bed at the other end of the house. That spoiled the fun and spontaneity more than once....lol.
That said. From a mom's point of view I would say you can decide how you feel and what you want to happen.
You can give them a deadline of when they need to be out.
You can decide to downsize, a lot actually, and tell them the house is being put on the market and that you expect them to be moved out when it sells.
You can decide to let them stay as long as they want, to welcome them into your home with open arms. Knowing as long as they are there your grandchildren will just be a few steps away and you will always be there for them.
It seems to me you have to resolve the issues that stand.
Why not make it standard to have a monthly household meeting to discuss any issues and make some simple goals. I would have a lot of chores in jars, daily, bi-weekly, weekly, bi monthly, monthly, every 3 months, every 6 months, once per year.
Then I would tell them that each adult was going to take turns picking out one job from each jar. That would be their assigned task. IF they didn't do it no one else would be, it would be their job.
Then let them start picking out those lovely jobs like empty the dishwasher after every meal, clean every toilet in the house each Monday and Friday, clean the carpets with a steam cleaner each march and September, etc..... They will get tired of getting all this work assigned.
They will want to move more after doing this one thing than any other thing. They won't think they'll have to work that hard in their own place.
Either way, you won't have so much to do.