loft conversion permitted volumes - external or internal?

[sss1978]

Just had another baby and thinking ahead, the roof is 100 years old and needs replacing and thinking about a loft conversion.

Just a little confused, the volumes under permitted development, 40 cubic meters for terrace houses, is that the extra usable roof space after it's been boarded or the external volume i.e including roof joists etc?

Thanks!

 

[ban-all-sheds]

Pretty sure it's external.

 

[tony1851]

As he said, it's only external measurements, ie the bit you see above the roof on the outside.

 

[sss1978]

Many Thanks, we have an L shape terrace, i.e a bit off the back with kitchen etc and I'm hoping to have it all built over, and using all the space would be over the PD limits, I'll have to get an architect in to measure up exactly and see what they can do!