--- title: "12 UK Extension Types Crossing Public Sewers (2026) — Part H4 Foundation & Lintel Checklists | BuildOverAgreement.co.uk" source_url: "https://buildoveragreement.co.uk/extensions" retrieved_by: "AnswerRail Edge Engine v1.1.0-parity-v20 (https://answerrail.com)" level_of_detail: "standard" token_payload_reduction: "93.1%" raw_html_bytes: 106541 compiled_body_bytes: 7369 entities_extracted: 4 currency_recall_static_html: "not_applicable" link_recall_static_html: "100%" monitored_content_links: 12 rfc_compliance: "RFC 9110" last_verified_at: "2026-10-17T13:29:46.440Z" cache_age_seconds: 0 origin_cache_control: "public, max-age=0, must-revalidate" --- > **Canonical Source:** [https://buildoveragreement.co.uk/extensions](https://buildoveragreement.co.uk/extensions) > > *2026 structural foundation bridging, invert depths, self-certification pass rates, and Part H4 checklists for rear, wrap-around, two-storey, and conservatory extensions.* ### Organization: BuildOverAgreement.co.uk (UK Drainage & Environmental Standards Desk) | Field | Value | | :--- | :--- | | **Organization** | BuildOverAgreement.co.uk (UK Drainage & Environmental Standards Desk) | | **Official Website** | https://buildoveragreement.co.uk | | **Email** | compliance@buildoveragreement.co.uk | ### Entity: BuildOverAgreement.co.uk — UK Part H4 Sewer & Foundation Calculator | Property | Detail | | :--- | :--- | | **Type** | WebSite | | **Name** | BuildOverAgreement.co.uk — UK Part H4 Sewer & Foundation Calculator | ### Entity: BuildOverAgreement.co.uk Part H4 Compliance & Lintel CAD Calculator (2026) | Property | Detail | | :--- | :--- | | **Type** | WebApplication | | **Name** | BuildOverAgreement.co.uk Part H4 Compliance & Lintel CAD Calculator (2026) | ### Step-by-Step Procedure: How to Obtain a UK Build Over Agreement for an Extension (2026) > **Objective:** Step-by-step statutory process under Building Regulations Approved Document H4 and Section 174 Water Industry Act 1991. #### 1. Locate Public Sewer & Invert Depth Consult your CON29DW drainage search or lift the inspection chamber cover to measure sewer distance, pipe diameter (100mm–300mm), and invert depth. #### 2. Check 3.0m Trigger Zone & 45-Degree Load Cone Verify whether your new extension foundation trench falls within 3.0m horizontally of the sewer or inside the 45-degree angle of repose from the pipe invert. #### 3. Specify Foundation Depth & Bridging Lintel Excavate foundations to at least sewer invert + 150mm, and span over or near the pipe using reinforced concrete lintels with 450mm end bearings and a 150mm compressible crown void former. #### 4. Submit Free Self-Certification or Formal Dossier Lodge your 1:20 cross-section, 1:100 site plan, and WRc MSCC5 CCTV drain survey with your statutory water undertaker and Building Control before pouring concrete. Architectural Typologies # 12 Common Extension Types Crossing Public Sewers Foundation bridging requirements, typical CCTV costs, and self-certification pass rates for standard UK residential extensions. Residential Extension & Groundworkshigh Risk ## [Single-Storey Rear Kitchen Extension](/extensions/rear-kitchen-extension) The single most common residential build-over scenario in the UK. Rear boundary sewers installed during Victorian, Edwardian, and 1930s suburban development run directly through the extension footprint. Typical Fnd Depth:1.2m Self-Cert Pass Rate:68% Party Wall Notice:Triggered [View Structural Checklist](/extensions/rear-kitchen-extension) Residential Extension & Groundworkscritical Risk ## [Victorian Terraced Side-Return Extension](/extensions/side-return-extension) Infills the narrow pathway between the original rear outrigger and the boundary line. Usually runs parallel to or directly over the main soil and vent stack drain serving multiple terraced houses. Typical Fnd Depth:1.4m Self-Cert Pass Rate:35% Party Wall Notice:Triggered [View Structural Checklist](/extensions/side-return-extension) Residential Extension & Groundworksmedium Risk ## [Conservatory Replacement with Warm Roof / Orangery](/extensions/conservatory-warm-roof) Replacing a lightweight UPVC conservatory with a substantial brick and timber solid-roof orangery often reveals unapproved historical construction over a public drainage asset. Typical Fnd Depth:0.9m Self-Cert Pass Rate:88% Party Wall Notice:Triggered [View Structural Checklist](/extensions/conservatory-warm-roof) Residential Extension & Groundworkslow Risk ## [Detached Garden Room / Home Office Outbuilding](/extensions/garden-room-outbuilding) Constructing habitable detached offices, gyms, or annexes at the rear of domestic curtilages near rear-fence trunk sewers. Typical Fnd Depth:0.8m Self-Cert Pass Rate:88% Party Wall Notice:Triggered [View Structural Checklist](/extensions/garden-room-outbuilding) Residential Extension & Groundworksmedium Risk ## [Integral / Attached Garage Conversion](/extensions/garage-conversion) Converting attached or integrated garages into habitable bedrooms or utility rooms. Often uncovers rainwater downpipe connections and shared front-drive sewers. Typical Fnd Depth:1.1m Self-Cert Pass Rate:88% Party Wall Notice:Triggered [View Structural Checklist](/extensions/garage-conversion) Residential Extension & Groundworkscritical Risk ## [Two-Storey Rear Brick Extension](/extensions/two-storey-rear-extension) Heavy two-storey masonry walls transmit over double the dead load of single-storey extensions, requiring deeper foundation footings that directly intersect sewer zone of influence. Typical Fnd Depth:1.8m Self-Cert Pass Rate:35% Party Wall Notice:Triggered [View Structural Checklist](/extensions/two-storey-rear-extension) Residential Extension & Groundworkscritical Risk ## [Wrap-Around Rear & Side Extension](/extensions/wrap-around-extension) Encloses both the side passageway and the rear garden footprint, creating a 90-degree bend over drainage assets where multiple laterals merge. Typical Fnd Depth:1.5m Self-Cert Pass Rate:35% Party Wall Notice:Triggered [View Structural Checklist](/extensions/wrap-around-extension) Residential Extension & Groundworkslow Risk ## [Loft Conversion with New Soil & Vent Stack Drop](/extensions/loft-conversion-soil-stack) Adding an ensuite bathroom or master suite requires dropping a new 110mm soil stack down the property exterior into the subterranean drainage network. Typical Fnd Depth:1.0m Self-Cert Pass Rate:88% Party Wall Notice:Triggered [View Structural Checklist](/extensions/loft-conversion-soil-stack) Residential Extension & Groundworkslow Risk ## [Front Porch / Cloakroom Extension](/extensions/porch-front-extension) Extending front entrance hallways often crosses water supply service pipes and front-garden surface water drains connecting into road gullies. Typical Fnd Depth:0.9m Self-Cert Pass Rate:88% Party Wall Notice:Triggered [View Structural Checklist](/extensions/porch-front-extension) Residential Extension & Groundworksmedium Risk ## [Raised Timber Decking / Porcelain Patio Over Drain](/extensions/patio-decking-over-drain) Constructing permanent hard landscaping, raised patios, or composite decking directly above public or adopted shared drainage runs. Typical Fnd Depth:0.6m Self-Cert Pass Rate:88% Party Wall Notice:Triggered [View Structural Checklist](/extensions/patio-decking-over-drain) Residential Extension & Groundworkshigh Risk ## [Self-Contained Granny Annexe / Ancillary Dwelling](/extensions/annexe-ancillary-building) Independent residential accommodation in garden grounds requiring dedicated foul and surface water connections into the public sewer network. Typical Fnd Depth:1.3m Self-Cert Pass Rate:68% Party Wall Notice:Triggered [View Structural Checklist](/extensions/annexe-ancillary-building) Residential Extension & Groundworkscritical Risk ## [Basement Conversion / Retrofit Subterranean Excavation](/extensions/basement-cellar-excavation) Excavating subterranean living spaces deeper than the invert of adjacent street and shared sewers, creating massive surcharge pressures and flotation risks. Typical Fnd Depth:3.2m Self-Cert Pass Rate:35% Party Wall Notice:Triggered [View Structural Checklist](/extensions/basement-cellar-excavation) ## Related UK Statutory, Planning & Building Regulations Calculators (Verified Edge Mesh) - **[RSJCalculator.co.uk](https://rsjcalculator.co.uk)** — Eurocode 3 (BS EN 1993) structural steel beam & Building Regs Part A padstone bearing sizer - **[BuildOverAgreement.co.uk](https://buildoveragreement.co.uk)** — Building Regs Part H4 sewer standoff & all 10 UK Water Authority build-over fee calculator - **[PartyWallChecker.co.uk](https://partywallchecker.co.uk)** — Party Wall etc. Act 1996 s.1/s.2/s.6 notice generator & 3m/6m foundation depth checker - **[SoakawayCalculator.co.uk](https://soakawaycalculator.co.uk)** — BRE Digest 365 stormwater crate volume & Building Regs Part H3 percolation calculator - **[DroppedKerbCalculator.co.uk](https://droppedkerbcalculator.co.uk)** — Highways Act 1980 s.184 vehicle crossing council fees & 4.8m driveway depth checker - **[GardenOfficePower.co.uk](https://gardenofficepower.co.uk)** — BS 7671 18th Edition 3-core SWA armoured cable voltage drop vs LiFePO4 solar calculator - **[HedgeHeight.co.uk](https://hedgeheight.co.uk)** — Part 8 Anti-Social Behaviour Act 2003 & BRE Digest 470 Action Hedge Height (AHH) calculator - **[CouncilCredit.co.uk](https://councilcredit.co.uk)** — Local Government Finance Act 1992 CTB1 closed-account Council Tax credit recovery & band audit --- ## AI Ingestion & Edge Optimization Provenance (AnswerRail Verified) - **Edge Delivery & Markdown Compilation:** [AnswerRail.com](https://answerrail.com) — Layer-7 AI Crawler Content Negotiation, `/llms.txt` & Byte-0 Schema Compiler - **Dedicated AI Markdown Subdomain:** https://ai.buildoveragreement.co.uk/ - **Cryptographic Ingestion Certificate:** https://answerrail.com/verify/buildoveragreement.co.uk - **Live Crawler & Referral Telemetry:** https://answerrail.com/stats/buildoveragreement.co.uk