City of London · London
Basement Extension in City of London
Limited residential stock; primarily apartment fit-outs and listed building work.
City of London
Basement Extension specialists across City of London.
We deliver basement extension projects across the full City of London postcode set — EC1, EC2, EC3, EC4. Limited residential stock; primarily apartment fit-outs and listed building work.
A basement extension adds new floor area below the existing house AND beyond the original footprint — typically extending under the rear garden, creating substantial new space for cinema, gym, spa, swimming pool, wine cellar or guest suite. It is the most ambitious and expensive basement build type. Planning approval is restrictive in most central London boroughs but achievable with the right design. Builderr delivers basement extensions in London at £200,000–£500,000+ over 28–36 weeks.
Every City of London project includes a free desk-based planning assessment before we quote. We know the local planning officers, party wall surveyors and approved inspectors by name.
Costs
Basement Extension cost in City of London 2026.
| Configuration | Cost range | Timeline |
|---|---|---|
| Standard basement extension, 50–65m² across house + garden | £230,000–£320,000 | 28–32 wks |
| Premium basement extension with light wells, cinema, gym | £320,000–£420,000 | 30–36 wks |
| Basement extension with pool or spa | £380,000–£500,000 | 32–40 wks |
| Inner London K&C/Westminster ultra-premium spec | £450,000–£800,000 | 36–48 wks |
Local advantage
Eight years of basement extension work in City of London.
Familiarity with City of London's planning officers, party wall surveyors and approved inspectors cuts weeks off the approval timeline. We deliver consistent quality across all 4 postcodes the borough covers.
Recent City of London work
Basement Extension examples.
Builderr vs other London builders.
The construction industry has a wide distribution of operators. Here's what changes between a directly-employed, fixed-scope outfit and the alternatives.
| Criterion | Builderr | Typical London builder | Cowboy outfit |
|---|---|---|---|
| Labour model | Directly employed team (PAYE) | Mixed subcontract gangs | Day-rate cash labour |
| Pricing | Fixed-scope itemised quote | Estimate + provisional sums | Verbal price + variations |
| Design & engineering | In-house architect + SE | Outsourced, separate billing | Builder draws on the back of an envelope |
| Planning + LDC handled | Yes — included in price | Often charged extra | Builder asks you to apply |
| Party wall surveyors | Instructed by us | Your responsibility | Skipped (illegal) |
| Building control | Plans + site inspections booked by us | Building Notice route | Not registered |
| Project management | Dedicated PM, weekly photo updates | Foreman doubles up | Owner-manager juggles 5 jobs |
| Payment schedule | Stage payments against signed-off milestones | Weekly invoices | Cash up front |
| Insurance | £10M PL + 10yr structural warranty | £2–5M PL only | No documented cover |
| Snags at handover | <3 typical | 20–30 typical | Walk-off mid-job common |
| Variation creep | 0% — fixed scope | +15–25% over original quote | +40%+ regularly |
Save £70,000–£157,500 on a basement extension in City of London.
Industry data (FMB, RICS, Which? Trusted Trader 2024) shows the average London construction project overruns by 18–22% on cost and 25–35% on time. Fixed-scope contracts with a single accountable team eliminate that variance. The savings above assume a typical project at £350,000.
FAQ
Basement Extension in City of London.
How much does a basement extension cost in City of London?+
Basement Extension projects in City of London typically cost £200,000–£500,000. City of London sits across postcodes EC1, EC2, EC3, EC4; local rates align with the wider London market.
Do basement extensions in City of London need planning permission?+
Limited residential stock; primarily apartment fit-outs and listed building work. We carry out a free desk-based planning assessment for your specific address before quoting, identifying whether permitted development, prior approval, or full planning applies.
Do you work across all City of London postcodes?+
Yes — we deliver basement extension projects across all City of London postcodes including EC1, EC2, EC3, EC4.
How much does a basement extension cost in London?+
A standard 50–65m² basement extension across house and garden costs £230,000–£320,000 in 2025. Premium spec with light wells, cinema and gym £320,000–£420,000. Adding a pool pushes total to £380,000–£500,000+. Inner London K&C/Westminster ultra-premium spec can exceed £800,000.
Can I extend a basement under my garden in London?+
In most London boroughs, yes — though restrictions apply. K&C limits extension to 50% of rear garden depth; other boroughs have similar policies. Front-garden basements are heavily restricted everywhere. Full planning permission and Basement Impact Assessment are mandatory. The viability of sub-garden extension is borough-specific and we assess at the initial consultation.
Get a City of London basement extension quote.
Senior consultant call within one business hour. Fixed-scope quote for your City of London project — no provisional sums.
Other types in City of London
Explore other build types in City of London.
Basement Extension across London