Can someone explain to me in simple terms what permitted development is?

[HeadFairy]

Would it for example include raising the roof at the back of our house?

They did change the rules recently but basically it means you are allowed to increase the size of your property by a given amount without needing planning permission.
Not very helpfully though I can't remember if that is foot print or volume of house. Or what the permitted increase is.

Has your house been extended before? If it has you may have used up some or all of these rights already.

There's a good guide on this website.

I don't think you can raise a roofline without planning permission though.

The house is unextended, the problem we have is the pitch of the roof is too low to do a straightforward loft conversion. We've been refused pp on our first go because the planners weren't happy with the effect on the street scene of raising the roof at the front, but I was wondering if raising it at the back would make a difference.

I'm afraid raising the roof isn't allowed under permitted development. Your architects should have known that. You can't raise at the front or back. You would have to do planning permission but I think you would have been very unlikely to get it for the reasons it isn't allowed in permitted development. You can only lower the ceilings in the floor below the loft I think.

Itshouldntmatter after we were refused I did speak to the planning case officer to try and sound out whether raising the roof, regardless of the design would be refused and he said he was sure a solution could be found. the two houses either side of ours both have roof ridge lines higher than ours and the plan was only to take the ridge line as high as the neighbouring properties.