Loft converstion with low butterfly roof

[Forster]

Anyone done this who can offer practical tips.

I think the only way you can go into the roof if you have one of these is to build a mansard. As these roofs are most often found on Georgian buildings that are likely to be listed or in conservation areas, this creates a whole raft of planning problems. Is the house part of a terrace or row where other owners have added mansards? If not, I'd say it was unlikely that you will be able to do it.