Do I need S104 for single house extension?

Usually no — typical extension connects to existing private drain (which connects to public main); no new shared sewer infrastructure created. Section 106 build-over agreement (if extending over existing public sewer) may apply; that's separate from S104. Only S104 if planning condition imposes (rare for single house).

Difference between Section 104 and Section 106?

S104 = adoption agreement for new sewers (developer creates sewer, Thames Water adopts on completion). S106 = build-over agreement for building near or over existing public sewer (extension over a sewer; protects sewer access during + after build). Both Water Industry Act 1991. Many large developments need both.

What if I refuse Section 104 adoption?

Possible — keep drainage private; all owners on shared drainage perpetually liable for maintenance + replacement; risk of dispute among multi-owner developments; substantial buyer concern at sale. Planning may not permit refusal — many LPAs require S104 as condition for residential development.

Thames Water Section 104 London | Builderr

What Is a Thames Water Section 104 Agreement in a London Renovation?

Thames Water Section 104 agreement: under Water Industry Act 1991 Section 104, developer can agree adoption of new sewers + drains by Thames Water on completion. For new dwelling / new estate / large extension creating new drainage: technical approval at design stage; bond / surety to cover defects; 12-month maintenance period; final adoption inspection. Cost £8,500–£35,000+ depending on scope. Often required as planning condition. Avoids private drainage liability indefinitely.

When Section 104 needed

S104 agreement triggered when: (1) new dwelling created (subdivision of plot, new-build, mews development); (2) new private sewers connect to public network; (3) substantial extension or change-of-use creates new drainage that future-owners may need adoption rights over; (4) planning permission condition mandates S104. Not needed for: routine extension connecting to existing private drains; single-dwelling refurbishment with no drainage change; minor drain re-route within existing private system. Single-plot residential: usually no S104 needed for typical house extension (drainage connects to existing private drain → public main); S104 only if creating new shared drainage with neighbour or planning condition imposes. Multi-plot residential (mews, courtyard development, terrace of new dwellings): S104 typically required to provide future-owners with public adoption of shared sewers.

Process

Stage 1 — Technical Approval (pre-construction): developer submits drainage design + calculations + soakage tests + materials specification; Thames Water Adopted Sewers team reviews + approves or requires amendment; £450–£1,800 application fee + £85–£350/hour technical review fee. Stage 2 — Bond / Surety: developer provides financial bond (£12,500–£85,000 typical) to cover defects + remediation if drainage fails during 12-month maintenance period; bond released on satisfactory adoption. Alternative: Performance Bond from insurer. Stage 3 — Construction Inspection: Thames Water inspector attends key milestones; CCTV survey final; air-test for water-tightness. Stage 4 — 12-month Maintenance Period: developer maintains drainage; Thames Water re-inspects at 12 months; defects rectified; final adoption sign-off; bond released. Stage 5 — Adoption: Thames Water takes ownership + maintenance liability indefinitely. Programme: 6–14 months from initial application to final adoption.

Cost + practical

Typical small development (2–5 dwellings new sewer ~50m): application + technical review £1,500–£3,500; bond £15,000–£35,000 (refundable); construction inspection £750–£1,800; final adoption £450–£950; CCTV survey £450–£950. Total upfront £18,000–£42,000 with £15,000–£35,000 returned at year 1 = net £3,000–£8,000. Larger development (10–30 dwellings, complex SuDS, attenuation tank): £25,000–£85,000+ total. Single-house extension typically excluded — connects to existing private drain → public main without new sewer infrastructure (handled by Section 106 build-over agreement instead; see [[thames-water-build-over-agreement-cost]]). Pre-application advice from Thames Water Developer Services: £450/hour; recommended for novel scope. Failure to obtain S104 where required: future owner liable for private drain maintenance + replacement (potentially £8,500–£45,000 per incident); discovered at conveyancing search + valuation.