Cantilevered First-Floor Office Extension

I have a solid-walled (225 mm brick-and-stone) single-storey outbuilding measuring roughly 6 m × 3.6 m. I now want to create an office/workspace above it by adding a independant supported timber-joist... (Budget: £250 - £750 GBP, Jobs: 3D Rendering, Architecture, AutoCAD, Building Architecture, Building Design, Home Design, Revit, Structural Engineering)