Apologies I have misread it! I thought the 10% was a lower figure for
terraced properties.

It is. Non-terraced properties seem to get 15% between 70m3 and 115m3. Mine
falls off the bottom anyway, though, hitting the 50m3 minimum. As an
Edwardian building, it has no restriction on permitted development rights
that has been common on more recent estate conveyances.

OTOH, I don't want to use too much permitted rights up. I'm thinking of a
dormer rear roof extension if it can be made to look OK. A quick back of
envelope calculation shows 20m3, so it'll be marginal and probably require
PP, especially if the lean to is constructed. I probably overcooked the
estimate on the conservatory so could probably squeeze both in.

AFAIUI Building regs will apply since it is an extension.

I still think not. Conservatories below 30m2 floor area appear to be exempt,
with loads of conditions that aren't too onerous. It needs to have a
transparent roof, 75% glazed walls, a proper external door between it and
the house, seperately zoned and controlled heating (if heated), below a
certain height etc.

Christian.