This depends on what you want to do. Some minor alterations and extensions, particularly to houses, can often be carried out without the need for planning permission. This is known as permitted development.
It often also depends on whether your house has been extended in the past. Even if you only want to put a conservatory on the back of your house, if your house has been previously extended, chances are you will need planning permission for the conservatory.
There may be local circumstances which might affect the need for permission, for example if there have been conditions applied to previous permissions (restricting or removing your permitted development rights), your property is a Listed Building or you are in a Conservation Area.
Problems (in the form of enforcement action) can occur if you do any work without permission which would ordinarily require it, including being taken to court in extreme cases, so please be sure before you go ahead with any work to your property.