New development in green belt on domestic curtilage?

[Rachel B]

I am looking for development sites for single dwellings in my local area but its tricky as our area is heavily within the green belt and even the outskirts of towns fall in the green belt in many places. I've identified several sites that have large gardens (domestic curtilage I believe) that would be perfect for new single homes. Our road is a long country lane with large spaced out houses. Its 3/4 mile from the village centre and well outside the area the council have designated for infill development within villages but in the last few years,  two properties have been given planning permission for new dwellings to the sides of them as a sort of infill, in the domestic curtilage. I can't find anything in the local plan that would justify why they would have been granted permission. I've tried to educate myself but it seems that green belt means no development. So I'm wondering if I'm missing something regarding planning within the domestic curtilage in the green belt? What justification could they have been granted planning under?  Any ideas please? 

Edited May 3 by Rachel B