If you live in the Green Belt I am sure you are aware how difficult it is to obtain planning permission for any sort of work.
However recent changes in the legislation regarding Permitted Development Rights (PD) which came in October 2008 have meant there is now more scope for extending your property without the need for planning approval even in the Green Belt.
Previous PD legislation related to volume of extensions, i.e once you had gone over 70 cubic metres any other works would require Planning Permission.
This element has now been scrapped.
The new legislation now relates to linear dimensions as opposed to volume, therefore even though you already have a rear extension that is already over 70 cubic metres it may not now stop you adding a loft conversion or other extension under PD rights.
It should be noted that the PD legislation is rather complex in many areas. Even though you will not need Planning Permission for some works you will still need Building Regulation approval.
I also strongly advise that if you are going to extend your property and are having a Building Regulation application lodged you should also apply for a Lawful Development Certificate.
This paper confirms that as far as the Local Authority is concerned the works you are proposing to carry out are lawful and comply to the legislation regarding Permitted Development.
Come the time when you sell the property the Lawful Development Certificate may be required by your purchaser.