Extension Plus Loft Conversion — Combining Projects to Maximise Your Home's Potential

The case for combining both projects into one programme:

• **1. Cost efficiency**
• Many costs are fixed regardless of whether you do one project or two:
• •Site set-up (scaffolding, skip, site welfare, temporary electricity and water)
• •Professional fees (architect, structural engineer, party wall surveyor, Building Control)
• •Disruption to the household
• •Project management overhead

Running both projects simultaneously typically saves £8,000–£20,000 compared to running them sequentially. The savings come from shared scaffolding (which can serve both the dormer and the rear extension in some configurations), reduced professional fee overhead, and a single phase of disruption.

• **2. Better design integration**
• Designing both the extension and loft conversion together allows a more cohesive result:
• •The staircase from the first floor to the loft can be positioned to work with the new first-floor layout created by the extension and any reconfiguration
• •The first-floor bathroom can be relocated as part of the package rather than being a constraint
• •The structural design for the extension and loft can be coordinated (some extension structures support elements of the loft conversion)
• •The utility routing (plumbing, drainage, electrics) can be planned holistically rather than each project working around the other's existing constraints

**3. Single phase of disruption** For families living in the property, one extended phase of construction — even if longer than either individual project — is typically preferable to two separate phases of disruption. The kitchen, bathroom, and staircase are disrupted once rather than twice.

A typical combined extension and loft conversion for a Victorian mid-terrace London house includes:

• **Phase 1 — Enabling and structural works**:
• •Scaffolding erected to serve both loft and extension
• •Demolition of existing rear utility and WC to clear the extension footprint
• •Extension foundation excavation and concrete pour
• •Extension blockwork walls erected
• •Structural opening(s) formed between the existing house and extension (typically a steel RSJ across the full width)
• •Loft: existing loft access hatch removed; temporary stair scaffold
• •Loft: structural floor beams installed
• •Loft: dormer or hip-to-gable structure built (if applicable)

• **Phase 2 — Shell completion**:
• •Extension flat or pitched roof constructed
• •Dormer roof and cheeks waterproofed
• •Extension bifold or sliding doors installed
• •Dormer windows installed
• •Loft rooflights installed (if applicable)
• •Extension walls rendered or clad
• •First-fix plumbing throughout (extension ground floor, loft bathroom)
• •First-fix electrics throughout

• **Phase 3 — Internal fit-out**:
• •Insulation throughout
• •Plastering throughout
• •Loft staircase installed
• •Fire doors installed throughout
• •Second-fix plumbing (loft bathroom, extension kitchen if relocated)
• •Second-fix electrics
• •Kitchen installation (if relocated to extension)
• •Bathroom installation (loft)
• •Decoration throughout
• •Flooring throughout

• **What the combined project adds to the house**:
• •A new open-plan kitchen-diner-living space at ground level (extension)
• •A larger first-floor landing (from the reconfiguration enabled by the extension)
• •One or two new bedrooms in the loft
• •A new full bathroom in the loft
• •Increased property value and significantly improved functionality

• Combined extension and loft conversion costs depend on:
• •The size of the extension (depth, width, single or double storey)
• •The type of loft conversion (Velux, dormer, L-shaped, hip-to-gable)
• •The extent of internal reconfiguration at first-floor level
• •The specification of kitchen, bathroom, finishes, and windows

**Approximate costs for a combined project (London, 2025)**:

**Single-storey rear extension (4m deep) + Velux loft conversion (1 bedroom only)**: £80,000–£120,000

**Single-storey rear extension (4m) + rear dormer (1 bed + bathroom)**: £100,000–£145,000

**Single-storey rear extension (5–6m) + L-shaped dormer (2 bed + bathroom)**: £130,000–£185,000

**Single-storey rear extension + mansard loft conversion (2 bed + 2 bath)**: £150,000–£220,000

**Double-storey extension + rear dormer (1 bed + bath)**: £145,000–£200,000

**What's included in these figures**: All trades (demolition, structural, brickwork, roofing, plumbing, electrics, insulation, plastering, carpentry, decoration, tiling). Client-supplied second-fix materials (kitchen units, sanitaryware, flooring, tiles). Structural engineer and Building Control fees.

• **What's excluded**:
• •Architect/drawing fees (typically £5,000–£15,000 additional for a combined project)
• •Planning application fees (if required)
• •Party wall surveyor fees (typically £1,500–£4,000)
• •Unforeseen structural issues

**Programme**: combined projects typically run 5–9 months on site (vs 3–5 months for an extension alone or 3–5 months for a loft alone — the combined project is faster than two sequential projects, but longer than either one individually).