Structural design for 8x10m attic roof over existing flat-roof

I'm looking to add a half-hipped pitched roof to an existing flat-roofed garage (8m x 10m), and to make the roof space habitable. I need somebody to design the main roof structure with timber and/or steel. It was always intended that this be done one day, so the existing structure should already be capable of taking the additional floor and roof load.

The finished roof surface is to be plain tiles. My preference is to have a single large room upstairs with side walls perhaps 1.2m high, but I can be flexible if that would be expensive.

The existing ceiling joists are sufficient to take any additional vertical load, but rather than being stretched in single spans between opposite eaves, they are split and supported part way by perpendicular steel beams. Therefore they cannot be used to resist any spreading load imposed by the new rafters, unless perhaps the joists can be tied to each other somehow, e.g. with steel plates. Presumably the only other solution is to use steel for purlins and/or other structural members.

Attachments:
Flat Roof Garage Plans As Built:
Single Storey Garage Plan As Built
Single Storey Garage Cross Sections As Built
Pitched Roof Garage Plans Proposed:
Proposed Garage Ground Floor Plan
Proposed Garage Elevations
Garage Roof Photos:
Existing Garage Photo
Proposed Garage Photo
Proposed Garage Staircase Detail Photo

Any designs must be compliant with UK building regulations with respect to strength, and include calculations that demonstrate this.

I have no particular budget in mind, nor timescale. Please provide estimates. I'm prepared to pay a fair price, and I will consider any proposal on its own merits.