So many aspects to consider which might affect whether you need planning permission, so always worth checking with a planning officer first. While you may well need PP, I doubt this would be too contentious (unless you are in a conservation area, for example) and could well be under permitted development as you are unlikely to be increasing the volume by much, but it also will depend on whether any of your "permitted development allowance" may have already been used on other extensions.

Strongly advise you to get some advice and perhaps get an architect or building surveyor to do some initial outline plans to doscuss with the planning officer. Also, look at other similar houses in your street/area and see if they've done something similar - if they have, chances are you will be allowed also!

I agree with the above