warm deck flat roof construction advice

Hi all, looking for some advice on a new build warm flat roof for a detached timber frame garden office that I want to build.
The roof is to be approx 7.9m x 3.8m using 170mmx47mm C16 joists at 400mm centres, I was going to 3x felt with chippings on the outside and I am intending to plasterboard the ceiling and fit recessed downlighters with led lamps on the inside. The building needs to be under 2.5m high to avoid planning permission.
I am looking for the thinnest overall construction for the flat roof to gain as much headroom inside the room as possible, I realise the thinnest option might not be the cheapest but I have now looked at so many different constructions and insulation types that I am now pretty confused and was hoping someone had negotiated a similar problem and could advise the best way forward. Thanks in advance.

Depends how warm you want to be. You could thermal plasterboard the ceiling, glass fibre insulation between the joists then 18mm ply then the felt.

A warm roof involves having 125mm of insulation above the joists.

A cold roof as described by MW above would save some thickness but would involve the hassle of venting the whole ceiling void.

Thanks both for taking the time to respond to my post.
I want to stick with the unventilated warm roof construction.
I know this might be a stupid question but would 50mm insulation on top of the joists and 75mm insulation between joists but fitted tightly at the top of the joists still work as a warm roof construction?
A friend has also told me about a product that has insulation, vapour barrier and timber face all in one, Thermaroof TR31. I have googled it and seems like I might be able to get a good u value with something like the 86mm product on top of the joists. Have either of you had any experience with using this product?
Thanks again

We use a ply faced insulation board when decking a flat roof. Both Celotex and Kingspan (thermaroof) do version.

know this might be a stupid question but would 50mm insulation on top of the joists and 75mm insulation between joists but fitted tightly at the top of the joists still work as a warm roof construction?

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I personally would be happy doing it this way provided you can convince the bco that no cold bridging/shadowing will occur through the joists.