Do I need a Thames Water build-over?

Yes if your extension is within 3m of a public sewer — covers nearly all London terraces. Check on Thames Water's online sewer-map portal at design stage.

Who designs the drainage?

Architect or structural engineer typically packages drainage into building regs drawings. Specialist drainage consultant for complex sites.

Can I connect to a soakaway instead?

Surface water yes (subject to ground percolation tests). Foul water must go to mains sewer — soakaways for foul are not permitted under building regs.

Does Builderr handle the Thames Water application?

Yes — lodged at design stage so approval lands before groundworks.

Quick Answer

How much does a drainage connection cost for a London extension?

Drainage connections for a London extension typically cost £1,500–£8,500. New manhole and connection to existing combined sewer: £1,500–£3,200. Re-routing existing soil/waste pipes inside new footprint: £800–£2,400. Thames Water build-over agreement (mandatory if building within 3m of a public sewer): £350–£700 application + £1,500–£3,500 CCTV survey and works. Lifting concrete drives for access: £400–£1,200.

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What's involved

Pre-build CCTV drainage survey to map existing pipework. Engineer's drainage design showing new manholes, gullies, vent stacks and slopes. Connection to existing combined sewer at the boundary. Thames Water build-over agreement if extension footprint crosses or is within 3m of a public sewer.

Thames Water build-over

Mandatory when building over or within 3m of a Thames Water public sewer (most London terraces). Application £350–£700. Pre-works CCTV £600–£1,200. May require sewer reinforcement (lining or replacement) £1,500–£4,000. 6–8 week processing time — apply at planning stage to avoid programme delay.

Hidden costs

Section 106 sewer requisition (if no existing connection): £4,000–£15,000+. Pumped drainage where gravity fall is insufficient (basements, low-lying rears): £2,500–£5,500 for pumping station. Sustainable drainage (SuDS) requirements in some boroughs add soakaways or attenuation: £1,500–£4,500.

Ready to get started?

Senior consultant call within one business hour. Free desk-based planning assessment. Fixed-scope quote — no provisional sums, no day-rate creep.