For example, your building work could take away some of their light or spoil a view from their windows. If the work you carry out seriously overshadows a neighbour's window and that window has been there for 20 years or more, you may be affecting his or her "right to light" and you could be open to legal action. It is best to consult a lawyer if you think you need advice about this.
You may be able to meet some of your neighbour's worries by modifying your proposals. Even if you decide not to change what you want to do, it is usually better to have told your neighbours what you are proposing before you apply for planning permission or before building work starts.
If you do need to make a planning application for the work you want to carry out, the council will ask your neighbours for their views.
If you or any of the people you are employing to do the work need to go on to a neighbour's property, you will, of course, need to obtain his or her consent before doing so.
A neighbour might take you to court if you are negligent or cause nuisance.