Start with the elevation, not the product list
Good replacement sashes should respond to the architecture already present. Pane proportions, meeting-rail position, horn shape, frame depth and mouldings can all change how the façade reads.
Record what survives
Original joinery is useful evidence. Measure or photograph mouldings, glazing bars, sash horns and the relationship between frame and masonry before deciding the replacement detail.
Improve performance discreetly
Modern glazing, draught proofing and security can be integrated without making the window visually heavy. The correct solution depends on the property, acoustic priorities, conservation context and the existing openings.
Use the survey to resolve the whole specification
Paint, ironmongery and glass should be considered alongside the timber profiles rather than added at the end. That is where a made-to-measure approach earns its value.