This is long but I want to tell you my experience of a combined household. Long story short - it worked well until my mum's increased frailty and the onset of dementia caused me to have a breakdown. My parents were lovely parents and grandparents and we had a lot of good times together although we should have put more boundaries in place to protect our own family time.
My husband and I sold our tiny 2 bedroom cottage and bought my parents large 4 bed house when our oldest son was 2 years old. The design of the house lent itself to being split in 2 and it worked well enough. We had 2 bedrooms, lots of living space, a bathroom and kitchen and mum and dad had one bedroom. We had our own front doors and gardens.
Our family grew and mum and dad eventually moved into a static home in the garden. Over the years mum's health deteriorated and I was able to help dad to care for her. My parents were very hands on grandparents and the children spent a lot of time with them.
My dad died 9 years ago and I became mum's sole carer. At that point it wasn't too onerous. My middle daughter would spend most of her free time with Grandma by choice and I already did all her cooking/washing/shopping and had done for years.
However it was quite a tie as mum was unable to do much for herself - couldn't make a cup of tea or even a sandwich - so if we needed to go away (DMiL) lived 4 hours away at that point - my cousin would have mum to stay with her.
Just over 2 years ago, in June 2019 mum became desperately ill with a perforated ulcer and sepsis. She spent a fortnight in hospital and was discharged with a reablement package but no assessment from OT to see if she could cope at all in her home and no adaptations such as a commode or a decent walking frame.
Before becoming ill mum's mobility was failing and she was afraid to walk from her armchair to the bathroom if there was no one with her but she managed to convince the reablement team that she didn't need help and they ended the package early.
Within a few weeks she was unable to get in and out of bed without help, she was less "with it" and more demanding. At the same time my middle daughter had started a new relationship and understandably wanted to be with her boyfriend rather than sitting with Gma every evening so the responsibility fell to me.
I was in contact with Social Services, the surgery, the local care agency - and had been since mum had gone in to hospital and begged for help. Eventually, at the beginning of March 2020 a care package was put in place - a carer twice a day to get her up and put her to bed. In the meantime I carried on doing everything else. And the pandemic hit. To start with it was ok - DH was furloughed, teen daughter was still at home, middle daughter and her now fiance were abiding strictly by the rules and didn't see each other (other than waving through the car window when he would leave a bunch of flowers at the bottom of the drive.
Then restrictions began to ease, daughter got married (1st in the county to get married after lockdown) and youngest DD went off to university
From then on I was trapped at home. I couldn't leave the house without a mother sitter and I couldn't spend more than hour in my house without her calling and needing support with something - anything from going to the toilet to changing the station on her tv. I was getting calls from her in the early hours of the morning complaining that the carers hadn't been and she was ready for lunch -even after I bought her a grow clock - she couldn't remember which was night and which was day.
She hated the carers and the routine that was imposed on her - going to bed at 9 p.m., getting up when the carer arrived without having her breakfast (from the day she got married she was used to my dad, then me, bringing her breakfast in bed), claiming that they did nothing to help her and that they were coming for my convenience not hers. She was actually quite vicious and I agreed to pay half the cost of the care package because it was of benefit to me.
Then in June last year she had a fall and was stuck on the floor of her bathroom. My husband and I were unable to move her and she had to wait 4 hours for an ambulance. They checked her over and couldn't find anything wrong but when the carer arrived she thought mum might have had a stroke. The GP told us to phone 999 again and tell them that she recommended she was admitted and that is what we did. The next day I ended up in hospital with amnesia and I am still struggling to be the person I was before it all happened.
So she had a few weeks on the medical ward and a proper OT assessment. We said that we would continue to provided meals and support but that we would not be involved in personal care at all. ie we could not be with her overnight and she needed to be able to take herself from the bed to the toilet safely. OT found she was unable to transfer herself and she went to the local rehab ward. Once there it was clear that cognitively she had deteriorated hugely. She couldn't remember how many children she had (one - me) and thought she had five (I was the one with five children!).
Social Services agreed that her home environment wasn't safe and that she needed 24 hour care so after a long stay in rehab she was offered a place in a lovely home and she moved in in October.
She is very unhappy, thinks she has been dumped, can't understand why she can't come home "to her kids" (all of whom are adults, none of whom live at home, even the youngest is getting married next year), and is so furious with me she will not make eye contact and half the time doesn't even remember who I am.
For 9 years mum was able to live in the home she loved but it took a huge sacrifice by my whole family. Knowing how it ended would I do it again? I honestly don't know. We had good times and I know I did everything possible to keep her at home. Our marriage survived - right now we feel almost like we are on honeymoon - and my relationship with my children did too even though I feel I neglected them in many ways.
If you decide to go ahead I would say that whatever else you do make sure you have separate living areas so that you and your daughter can spend quality time together. If you are buying together you need to consider the implications if she needs care (my mum is fully funded by the council because she has no assets at all - we bought the house at half the market value - just enough to cover the mortgage) and remember that there are three people to consider here and one of them is you.