Asking someone with a messy house/ garden to clean up?

[GeordieDownSouth]

My mum wants to sell her house. She lives on a lovely cup-de-sac and her house is gorgeous.

However, her neighbors house (2doors along) is a mess. The front garden is full of rubbish, including a moudly old sofa. One of the windows shows a room piled high to the ceiling with junk.

Up to now, my mum has been "live and let live" about the situation. A woman, man, and two male teenagers live in the house, and she says they're not approachable.

When she had estate agents round, two of them mentioned that the eyesore house may reduce the value of her house! Sadly I agree.

Does anyone have any ideas of how to approach this issue with the neighbors?

Yes, try this. Frame it as a neighbourly offer to help. Potential buyers will notice the didactic in the garden - they probably won’t be looking into their windows, so see if you can at least ‘help’ them to clear the garden.

Didactic!!! Meant sofa etc!!

A friend of mine used to work in mental health/occupational therapy and did home visits. She commented that the house with the sofa in the garden was the one her clients were most likely to be living in.
Contact your local Council - there may be hazardous elements to the hoarding but possibly also people unwell/vulnerable.

As someone who currently struggles with an overgrown garden, someone offering help would be very welcome.

They may genuinely have no money for a skip.