To offer to clean for my adult daughter

If its caused arguments in the past, do you think you're in danger of this driving a further wedge between you?
Have you ever discussed the potential ADHD with her? Supporting her getting diagnosis, particularly if you can afford to pay for her to go private, and getting medication might mean she can start to do something about this long term.

If it's like my nephew it's the awareness it needs doing and the planning to do it not the actual doing of it. So going to a launderette still wouldn't happen.

Can you pay for a cleaner for her, or better yet for both DDs? The hands-off approach of it being a different person might be easier for her to take.

is there anyone else, an aunty, that might have a more relaxed relationship ?

I don't think you're being unreasonable but I imagine that she could take offense if she knows her sister told you her house was so filthy that you feel compelled to go clean it for her. Could you just arrange a visit yourself or tell her you're going to be in the neighborhood and then discuss it when you see it for yourself?

Are you sure it’s just ADHD? That she doesn’t suffer from something else on top of it, such as depression?
You chowing it as a one off would be a start but essentially like putting a band aid on an open fracture….. It’s not just about the mess and what’s socially acceptable but if she’s neglecting personal hygiene and not brushing her teeth etc. then that will have serious negative effects on her health long term.

Or visit without saying anything and then just do it without saying anything if you think this is better approach.

Is there bad feeling between you on this topic due to her teen years?
If not then I think it’s a fantastic idea and emotions aside if you could do that for her 2 or 3 times a year it would be life changing for her.
I think the best way forward is to get your other dd who stayed there to say to her something like “Im going to call mum round to clean and tidy for you, she’s got nothing to do anyway and it will keep her busy”.
This may make it easier for daughter to accept the help.
I do know another similar situation where mum cleans and tidies once a week and this is much appreciated without any weird vibes.
Do let us know how it goes.

Can she afford to take her clothes to the laundrette? I think that may be a good solution to blast through the backlog. Yes there's a risk some clothes may get ruined but round here you can drop off a bag of clothes and they'll then wash them for you and hand the bag back with clean folded clothes.
If she works, could she get a cleaner ? Then you could go and help sort out and a weekly clean may keep it going for her?

Do that and then arrange for a weekly cleaner.
She pays.

I would not offer. You can have a discussion about ADHD and whether she should get a private diagnosis (wait times horrendous otherwise). She then knows what she is dealing with. If she has ADHD you can then have a broader discussion to see if there is anything she would like help with. However there may well be a large dose of shame involved with the mess and she probably needs to find her own way.

If it was my daughter I’d do the same, although I’m not sure how you could bring it up. Maybe call her and say you’re planning on coming to visit and then discuss it when you get there? You don’t want to say that your other daughter told you how messy it is because that could drive a wedge between them.

I think this is one of these times where you might just have to turn up and do it regardless of how she feels about it. Sometimes we just have to make stuff happen in their best interests and this feels like time for an intervention imo.

then, once at a good starting point perhaps you can work through a simple list with her to help her try and keep on top of things?

how do you think she would take it?