When a double-storey makes sense
Single-storey extensions excel at delivering kitchen-diner space; loft conversions deliver bedrooms; double-storey extensions deliver both in a single project. For families who need an extra bedroom and bathroom alongside more kitchen space, a double-storey is usually the most efficient route. It also avoids the disruption and cost of doing two separate projects in sequence. Common scenarios: a semi-detached family home where the existing first floor offers only 3 bedrooms and the family needs 4; a terraced family home where the existing first floor offers only 2 bedrooms; a detached home where the existing rear is shallow and the new extension can match the depth across both storeys.
Planning rules
Double-storey rear extensions can fall under permitted development on detached and semi-detached homes if they stay within 3m depth, don't exceed the original ridge height, and sit at least 7m from the rear boundary — fairly restrictive limits. Most useful double-storey extensions therefore go via full planning. Approval rates vary by borough: Outer London (Bromley, Croydon, Sutton, Barnet, Enfield, Bexley, Havering, Hillingdon) is generally more permissive; Inner London (Hackney, Camden, Lambeth, Islington) more cautious about scale and visibility. Conservation areas almost always require redesign or refuse double-storey work outright. We pre-app with the borough on every double-storey project to confirm acceptable form before submission.
Structural method
Double-storey extensions are essentially small two-storey buildings tied into the existing house. Foundations are typically mass-fill concrete to 1.5m depth (deeper on clay with trees); strip foundations for the side walls, pad foundations for any internal corners. Walls are cavity brick-and-block, matching the existing fabric externally. The first-floor and roof loads transfer via steel beams across the openings at ground and first-floor level. Roof can be flat (cheaper, less visible from front) or pitched (more residential character, often required by planners). The junction with the existing house is critical: we tie new brickwork to existing with stainless steel tie bars and form continuous DPC, cavity tray and weep details to prevent water bridging the cavity.
First-floor layout
The upstairs of a double-storey extension typically delivers one large bedroom or one bedroom plus an ensuite. On wider properties (5m+ wide rear) we can fit two bedrooms with a Jack-and-Jill bathroom. The new room sits over the new ground-floor extension and is accessed via the existing first-floor landing — sometimes requiring a short corridor extension or repositioning of the landing wall. Window placement matters for both daylight and neighbour privacy: side windows usually need obscure glazing or restricted opening to comply with the borough's privacy guidance.
Ground-floor integration
Most double-storey extensions are combined with the removal of the existing rear wall to open ground-floor space into the new extension. This delivers a kitchen-dining-family layout matching what a single-storey rear extension would offer. The steel beam over the opening is sized to take the new first-floor loads plus the existing first-floor wall above, which means it's heavier than a single-storey extension equivalent — typically 305 UB or 356 UC depending on span. We always model the existing house's foundations during structural design to confirm capacity for the increased loading; on Victorian terraces with corbelled brick footings this sometimes requires localised underpinning.
Roof options
Pitched roof matching the existing house pitch and finishes is the planner's preferred option for character matching. Flat roof is cheaper, easier to detail, and gives a flatter modern profile — often acceptable in less conservation-sensitive boroughs. Hybrid roofs (pitched at the front of the extension matching the house, flat at the rear) work where planning preferences vary across the property. We model all three options at design stage and present each with its planning risk.
Cost dynamics
Per square metre, a double-storey extension is more efficient than two single-storey projects done in sequence. Roughly: a single-storey 25 m² rear extension runs £2,800–3,500/m²; a double-storey extension delivering 50 m² across two floors runs £2,400–3,000/m². The double-storey saves on foundations (one footprint, two storeys of space), planning fees (one application), and disruption. The trade-off is that the first floor of the new extension affects neighbouring privacy and amenity more than a single-storey, requiring more careful design and pre-app consultation.
