Should mum pay for the granny annexe?

I think you approach it from an angle of "we would love you have you live with us but unfortunately we cannot fund further building works" and let her decide if it's worth the money to her to pay for it.

You may need legal advice in case it comes under inheritance within a certain period of her death (sorry to upset you and I genuinely dont know if gifts come under this or how to protect yourselves if it does).

The important thing is that it has to be her decision to want an annexe and to want to pay for it. If you arent comfortable having her move in without this she doesnt move in.

I think she should pay as she will have cash on the hip and the benefit of living with family support.

IMO, in moral terms inheritance isn't due until her life is over so this cost is not early inheritance, it is an ongoing cost of living. She will be saving in tv licence, council tax, supported living etc. So morally she should pay so you just need the legal cover.

This will be the cleanest way.
set the rent at highest reasonable rate so you recoup the outlay in shortest time frame (inc interest on any loans if needed / agreed)
After that charge for utilities and food shopping and sundries.

If she pays for the extension things can get quite messy.

In situations like this its common for siblings to announce that "recollections vary" when she eventually passes and thing can get ugly quickly. It's best to ensure its as clean and transparent as possible.

The main issues I think are her estate having a claim in your assets - whether that is in terms of needing care in old age or after she eventually passes away - so you would want professional legal and financial advice before proceeding.

But no, you should not be out of pocket for helping your dm

It's your house so you should be paying for any work or upgrades to it.

But I'd think long and hard about having an elderly parent moving in with you full-time. Are you willing and able to provide care for her? If not, are you happy with carers coming in multiple times a day?

What an undertaking . Your DM is quite happy to do this to you?
Can't she move closer and buy in help? I've seen this situation unravel with a couple of friends and they both ended up with bitter regrets on antidepressants.

Pay for it yourself.

I wouldn't charge rent to her as such. Just maybe a contribution to utilities. She'd be getting cold weather payments etc which could be used.

Then she can sell her house and use it for care if needed and then split the inheritance equally if there's any left.

Is your mum of sound mind? Speak to her. I think she should pay. It's not like you need an extra kitchen and bathroom. I've known lots of people sell their property and go in on a bigger property with their dc. The money would disappear with care fees anyway.

It's quite an undertaking for you. Will your siblings help at all?

I think if she did go into a home eventually they would want to see where her house proceeds have gone.

What happens if she requires nursing care and she owns part of your home ie the extension?

It could get complicated if she's contributed to actual building costs. The annexe is your property, so it could be used by your mum, then another tenant. Keep it that you recoup the cost of the extension with rental payments that could be lower than market rate if you can afford it.

Yes I agree with pp, you pay she pays rent. Then siblings can’t get upset about it, but she’s contributing going forwards.

No experience but in my mind to avoid issues with siblings I would pay for the extension myself and then get parent to pay a fair rent.

You pay for it and she pays you rent.