Single story extension

Hi to answer your question it would be appropriate to instruct a architect to design your extension and if a steel box frame is required then this would be designed by a structural engineer while planes for said extension is being drawn.

In short, yes it is possible and no the construction method does not need to be a steel box albeit that wouldn’t be a bad idea. Things to consider: Survey and Analysis: Before any construction starts, perform a full structural survey of the existing property to determine its structural integrity. Ensure you understand the depth and type of foundations in the existing house. No Fines houses might have shallower foundations than modern standards. Foundations: The foundation for the extension should be designed such that it doesn't impose any undue stress on the existing structure. A common approach is to have a trench-filled foundation that is deeper than the existing foundation, often with a slip membrane or expansion joint to ensure there's no direct load transfer onto the existing structure. Wall Junction: At the junction where the new extension meets the no fines wall, insert a vertical damp proof course (DPC) to prevent moisture from traveling across. Create a physical and airtight seal between the new and old structures. This can be done using masonry ties or connectors that are drilled and fixed into the existing wall and embedded into the new wall as it's constructed. Insulation: No Fines concrete has different thermal properties than traditional constructions. Depending on your chosen construction method for the extension, there might be a need to address the thermal bridge at the junction. The insulation layer of the extension should seamlessly integrate with the existing structure to avoid cold spots or condensation risks. External Finish: Depending on aesthetic preferences, the external finish of the extension can either match the existing finish or contrast it. In either scenario, ensure the junction between the two finishes is watertight. Ventilation: Ensure that the extension has adequate ventilation, especially if there's a risk of moisture buildup at the junction with the existing structure. Services and Utilities: When integrating services (electrical, plumbing, etc.) into the extension, be cautious when drilling or chasing into No Fines walls, as the aggregate structure can be less predictable than traditional brick or block walls.