Depends to some extent whether by 'mess' you mean it is full of weeds and neglected, or full of rubbish and detritus.
There is no legal method except through planning laws if the 'mess' includes things which have been constructed, or if the person is a tenant, you can approach the Housing Society or the private landlord who can bring pressure to bear.
The only other thing to do is to talk nicely to your neighbour, perhaps offer help in weeding or clearing up. If the garden is a mess because the person is elderly there are people who will garden for them in return for a little of the produce, if a veg. garden.
Unfortunately, at the end of the day not everybody enjoys gardening like we do, and every person has the right to leave their garden in whatever state they wish.

It's a common problem, I'm afraid, Oldboo. Other than, as Bertie suggests, offering help, which may just be regarded as criticism and therefore offensive, there really isn't a lot you are allowed to do. If maybe you could get some of your other neighbours to make gentle representation to her, too, so that she doesn't think it's only you who has the problem, that might help. Good luck.

i used to feel i was the neighbour your talking about lol it wasnt bad just used to let the grass grow while i was pregnant last year, but now i definately do the most work on mine-with two kids too! it is a hard one, i would be worried about offending them! maybe subtly talk more about gardening to her and maybe find out why its a mess in general then you might be able to come up with some solutions. i suppose gardens that are nice belong to people who like gardening, your neighbour could have other reasons or just doesnt like gardening. hope you get somewhere!! all the best :)

maybe you could take it over, like a sort of alotment, or persuade someone else to. I have a friend who lets someone else grow veg in hers (for a share of the produce) so they are both happy. Depends on your relationship with the neighbour outside of the garden thing i suppose

great ideas above but the bottem line realy is if its not directly afecting you then theres nothing you can do . theres nothing like living next to bad nieghbers either .

Are you friendly?.....if so, offer to have a sort of party where some food is provided, and you all muck in and do the food and the garden. Get to know one another better and enjoy yourselves. Maybe you could call it a Jubilee garden and plant red/white and blue flowers. We have done something similar twice and it worked out well.

What a lovely idea.

If it is a council or housing association house then there will be term and conditions on their let to maintain the garden. If you know whom it is then phone and complain to them. They can revoke tenancy if they fail to look after the garden.

Private houses vary but there are usually terms in the deeds that say that gardens should be well maintained. It is almost impossible to enforce these without expensive legal action.

i tried to get my mates round for a 'work party' once like i think dorjac was suggesting but they wernt too keen maybe i should have missed out the work word hah!

its idealistic hoping you can all have a party as people who want a scruffy garden dont care but will realy care if you get the council ingvolved . id ask nicely and if nothing happens put 6` trellis on your fence and grow some clmbers if it offends you that much .

It's a sad point, Nosey, but weed seeds and vermin aren't stopped by a 6' fence. :o(

it`s a sad case but like mentioned not all people are gardeners,i have cleared both my neighbours gardens to a managable level on more than one occassion, but alas to no avail,spent one full saturday this year cutting a four foot clearing of six foot tall blackberries away from my fence [must admit the task gets a little easier every other year, as more and more roots come out] and the other side she tries to keep her grass down but it doesn`t stop the roots from growing under the gravel boards. in saying that i`ve had polite words,[on deaf ears mostly] but i don`t want to fall out with either side.hence i`m in the same dilemma as yourself and no answer best of luck for a solution

it is a real shame but a lot of people just dont care .

Do be careful. Many, many years ago, my Father (never a tactful man at the best of times) had a bit of a go at his neighbour over the garden wall at the state of her garden and how he was fed up at all the weed seeds it was generating. The next thing he knew, the husband was raging up to the front door, threatening to deck him for upsetting his wife, and we pretty much had to pull them apart. My 80 year old Dad would have taken him on, too. It didn't do a lot for an air of neighbourliness in the road.

i agree i mean look at it from this point of view . how would you like it if the nieghbers came round and asked you to make it untidy as they dont like it smart ?

my cottage style garden was commented on as being 'untidy' as my plants werent in neat rows, like the regimented ones next door. not a weed in sight, i dont think they dared tried to grow. but to me it was souless compared to what I liked. Must say I didnt take the comment too kindly;o)

I'm not surprised!

If you don't get on with the neighbour, let it ride. The best weapon is a tall fence within rules, as we did at the back many years ago to stop his alsations coming into our garden, after HIS fence collapsed. No more bother. Festering over who pays does you no favours. There are so many styles of gardening. Grass only for those who hate it but can stir to a bit of mowing. No dumping of appliances or other rubbish...by law and snitachable . Our party clearing efforts were a success, but maybe a bygone era? Only with friends.

sad but true .

What a fabulous, self-explanatory word, Dorjac, "Snitchable". Should be in the OED. Very useful, too. :o)