Modern Rear Extension with Bifold Doors

I'm looking for a skilled Structual Engineer to come around and tell me what size RSJ i need and other information i may need to create a modern rear extension on my property, featuring just under 6-meter aluminum bifold doors... (Budget: £20 - £250 GBP, Jobs: 3D Rendering, Architectural Engineering, AutoCAD, Building, Building Architecture, Construction, Construction Engineering, Interior Design, Project Management, Structural Engineering)