Estimated cost (P50)
£210,707
P20–P80 £178,164–£243,248 · £3,454/m²
Finish month (Balanced phase)
Month 20
Target month 20
Planning route
planning required
risk High · 3 rules triggered · Householder planning application
Heating running cost
£226/yr
Indicative band B · 3,357 kWh/yr
Embodied carbon (A1–A3)
23.0 t
377 kg/m²
Structure
Heavy steel needed
Opening 5.4 m exceeds 4 m: heavy steel with padstones; structural engineer required.
Violations
- budgetEstimated cost £210,707 exceeds budget £165,000 by £45,707
Reality Score
64
affordability34
planning50
sustainability100
complexity60
timeline100
Planning reading
| Rule triggered | Reading |
|---|---|
| Two-storey rear extension projects 4.2 m (> 3 m limit, Class A(f)). | required |
| Conservation area: two-storey or side extensions are not permitted development. | required |
| Two-storey rear extension: PD only if ≥ 7 m from rear boundary and materials match (Class A(h)). | uncertain |
Cost breakdown
| Package | Quantity | P50 | P20–P80 | Phase |
|---|---|---|---|---|
| Preliminaries, design & fees Design, structural engineer, building control, insurances | 9 % of works | £13,180 | £11,203–£15,157 | planning |
| Demolition & strip-out Strip-out of rear wall and affected fabric | 25.5 m2 | £4,570 | £3,427–£5,712 | planning |
| Substructure (foundations & slab) Ground-bearing slab, insulated; ground risk class C | 35 m2 | £12,342 | £9,257–£15,428 | substructure |
| External walls Cavity wall, standard insulation | 93.7 m2wall | £26,984 | £22,937–£31,032 | superstructure |
| Upper floors Joisted upper floor | 26 m2 | £3,994 | £3,395–£4,593 | superstructure |
| Structural steel / beams Opening 5.4 m exceeds 4 m: heavy steel with padstones; structural engineer required. | 1 item | £12,365 | £9,274–£15,456 | superstructure |
| Roof structure & covering Flat warm roof | 35 m2roof | £7,997 | £6,797–£9,196 | roof |
| Windows & external doors Aluminium double glazed | 8.9 m2glaz | £9,539 | £9,062–£10,016 | roof |
| First fix (electrics, plumbing, carpentry) Electrics, plumbing, carpentry | 61 m2 | £10,931 | £9,292–£12,571 | firstfix |
| Heating system Gas boiler + radiators (extension of system) | 1 item | £2,957 | £2,513–£3,400 | firstfix |
| Plastering & second fix Finish tier: good | 61 m2 | £13,469 | £11,448–£15,489 | secondfix |
| Kitchen Kitchen, good tier | 1 item | £17,920 | £17,024–£18,816 | secondfix |
| Bathrooms 1 bathroom(s) | 1 count | £9,568 | £8,133–£11,003 | secondfix |
| Decoration & floor finishes Decoration & floors | 61 m2 | £8,530 | £7,251–£9,810 | secondfix |
| External works & landscaping Patio, paths, planting | 55 m2ext | £5,280 | £3,960–£6,600 | external |
| Contingency | £15,963 | |||
| VAT (20%) | £35,118 | |||
| Total (P50) | £210,707 | |||
Timeline breakdown
Unconstrained build programme 13 months · funded finish month 20 on the Balanced phase pathway.
| Phase | Duration | Starts | Ends | Waiting for funds | Phase cost |
|---|---|---|---|---|---|
| Planning & preliminaries | 4 months | Month 0 | Month 3 | None | £23,430 |
| Substructure | 1 month | Month 4 | Month 4 | None | £16,291 |
| Superstructure to wall-plate | 2 months | Month 5 | Month 6 | None | £57,213 |
| Roof & weathertight | 1 month | Month 7 | Month 7 | None | £23,148 |
| First fix | 2 months | Month 8 | Month 9 | None | £18,332 |
| Second fix & finishes | 2 months | Month 15 | Month 16 | 5 months | £65,323 |
| External works | 1 month | Month 19 | Month 19 | 2 months | £6,970 |
Carbon breakdown (embodied, A1–A3)
| Element | kgCO₂e | kg/m² GIA | Share |
|---|---|---|---|
| walls | 8,901 | 146 | 39% |
| substructure | 4,200 | 69 | 18% |
| fitout | 3,660 | 60 | 16% |
| roof | 1,575 | 26 | 7% |
| steel | 1,400 | 23 | 6% |
| floors | 1,170 | 19 | 5% |
| external | 1,100 | 18 | 5% |
| glazing | 979 | 16 | 4% |
| Total | 22,986 | 377 |
Operational heating: 668 kgCO₂e/yr at 3,649 kWh delivered.
All figures are indicative concept-stage ranges from seed reference data. Planning classifications are rule-based readings of the GPDO householder classes, not advice; the energy band is an indicative estimate, not an EPC; structural triggers flag where an engineer is needed. Nothing here is a quote, a lending decision or professional advice.