Up until April 2008 it was essential to apply for planning permission before having solar panels fitted. However as the government looked to encourage people to buy into renewable energy they removed the requirement of planning permission to make it easier to install solar panels.
The only requirement which must be fulfilled when fitting solar panels is that the panels should not be installed above the ridgeline and should project no more than 200mm from the roof or wall surface.
If you live in a a-listed building or a conservation area then you will need to gain planning permission before installing. You should check with your installer before agreeing to have the work carried out.