St leonards Loft Conversion Structural Plans

My top–floor flat in TN38 0JH is just under 45 m² and I want to upgrade the existing 50 mm × 50 mm ceiling joists so the space above can serve both as dependable storage and, if the numbers work, a fully inhabitable room... (Budget: $250 - $750 USD, Jobs: Architectural Engineering, AutoCAD, Building Architecture, Building Design, CAD / SolidWorks, CAD/CAM, Civil Engineering, Construction Management)