Will I need planning permission?

In most cases, no. As long as the solar panels are within 20cm of the plane of the roof, and do not protrude above the ridge line, then the installation counts as 'permitted development', and no planning permission is needed. This is true (since an amendment to the Town and Country Planning Act was passed in October 2008) even for most installations in conservation areas, and even when the installation is on the principal elevation of the building and is visible from the highway. Council planning officers have been known to be unaware of the correct legislation - so if you get a difficult one, point them to the link above!

There are still a few cases where permission may be necessary, particularly on listed buildings.