It depends on how far the dwelling will be from your house, whether there is the possibility of double storey, how large the garden will be, the access and whether it will impinge on your privacy and whether or not they could plant trees on the border restricting your access to light - and the price!
Also whether you would be glad to be rid of part of the garden if it becomes too much to cope with in the future.
It would have to be a great deal more than £2,000 for us to sell. Why is the offer low? Stand firm on price if you decide you want to go ahead. (As DS says - always get three valuations.)