Hi,

I'm just coming to the end of a loft extension at my property in SE23, the cost does vary hugely depending on the spec that you go for but for a standard sized loft extension I'd budget for 40/45k including a bathroom suite, tiling etc and vat. Lewisham council have been easy to deal with and any reputable building firm will probably take care of that side of things along with party wall agreements etc.

Haven't gone for the side return but I have two friends that have, one opted for full width doors to the garden, the other didn't and regretted it.

Hope that helps!