--- title: "Integral / Attached Garage Conversion Over a Public Sewer (2026) — Part H4 Lintel & Build Over Calculator | BuildOverAgreement.co.uk" source_url: "https://buildoveragreement.co.uk/extensions/garage-conversion" retrieved_by: "AnswerRail Edge Engine v1.1.0-parity-v20 (https://answerrail.com)" level_of_detail: "standard" token_payload_reduction: "89.5%" raw_html_bytes: 138273 compiled_body_bytes: 14510 entities_extracted: 7 currency_recall_static_html: "90%" monitored_currency_tokens: 21 link_recall_static_html: "100%" monitored_content_links: 10 omitted_dynamic_states: ["non_default_tab_panels", "volume_slider_tiers"] conversion_notes: "Origin renders alternate tabs and volume pricing sliders via client-side JavaScript; serving default static AST state." rfc_compliance: "RFC 9110" last_verified_at: "2026-10-17T13:30:20.997Z" cache_age_seconds: 0 origin_cache_control: "public, max-age=0, must-revalidate" --- > **Canonical Source:** [https://buildoveragreement.co.uk/extensions/garage-conversion](https://buildoveragreement.co.uk/extensions/garage-conversion) > > *2026 UK Build Over Agreement & foundation lintel guide for a integral / attached garage conversion. Typical trench depth 1.1m, 88% self-cert pass rate, and 1:20 Part H4 CAD sizer.* ### 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. ### Frequently Asked Questions (FAQ) **Q: Do I need a Build Over Agreement for a integral / attached garage conversion?** > A: Yes, if any part of your integral / attached garage conversion foundation falls over or within 3.0 metres horizontally of a public or shared sewer. Across the UK, 88% of projects of this type qualify for fast-track self-certification when there is no internal manhole. **Q: How deep do foundations need to be for a integral / attached garage conversion near a drain?** > A: Typical foundation depth is 1.1m, or at least the sewer invert depth plus 150mm when excavating inside the 45-degree zone of influence under Approved Document H4. **Q: Does a integral / attached garage conversion over a sewer trigger a Party Wall Notice?** > A: Yes — excavating deep trenches within 3.0m of a neighbouring structure typically triggers a Section 6 Party Wall etc. Act 1996 notice. [Back to All Extension Types](/extensions) Residential Extension & GroundworksConflict Risk: **medium** # Integral / Attached 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% Typical CCTV Cost:£245 Party Wall Trigger:Mandatory Live Sizer ## Size Bridging Lintels for Your Integral / Attached Garage Conversion 01 Check 3m / 45° RuleFree Live 1:20 Cutaway 02 Verify £0 Self-CertOr Formal Water Board Route 03 Download 4-Sheet CAD Pack£24.99 vs £450 Architect 04 Book Pre-Build CCTVOr Submit to Water Portal Interactive Part H4 Sewer & Foundation Sizer ## Click a Real-World Scenario Preset or Enter Your Drain Measurements Below Planning a New ExtensionSelling / Buying a House MOST COMMONACTIVE Rear Extension Over Patio Drain New kitchen/dining extension wall crosses directly over a 150mm shared rear garden drain. NEAR DRAIN Building Near Neighbour's Drain (1.2m) Foundations sit 1.2m away from a shared sewer pipe without crossing directly over it. CRITICAL RULE Manhole Cover Inside New Room Existing drain inspection cover would end up inside your new extension floor. HOUSE SALE Selling House — No Agreement Buyer's solicitor flagged a past extension built over a sewer without water board consent. CAD Parameter Input Console ### Your Drain & Extension Details 1\. Your Regional Water Company Thames Water — Greater London, Thames Valley, Surrey, Berkshire, Oxfordshire, Gloucestershire, WiltshireSevern Trent Water — West Midlands, East Midlands, Warwickshire, Leicestershire, Nottinghamshire, Derbyshire, StaffordshireUnited Utilities — Greater Manchester, Merseyside, Lancashire, Cheshire, CumbriaAnglian Water — Cambridgeshire, Norfolk, Suffolk, Essex, Lincolnshire, Northamptonshire, BedfordshireYorkshire Water — West Yorkshire, South Yorkshire, North Yorkshire, East Riding of YorkshireSouthern Water — Hampshire, West Sussex, East Sussex, Kent, Isle of WightWessex Water — Bristol, Somerset, Dorset, Wiltshire, Bath and North East SomersetNorthumbrian Water — Tyne and Wear, County Durham, Northumberland, Tees ValleyDŵr Cymru Welsh Water — Cardiff, Swansea, Newport, Wrexham, Gwent, Glamorgan, PembrokeshireSouth West Water — Devon, Cornwall, Isles of Scilly, Parts of Somerset and Dorset Self-Cert Fee: **FREE (£0)**•Formal Class 1: **£343** 2\. Distance: New Wall to Drain Pipe0.00m (Directly Over) Over (0m)Near (0.8m)Close (1.8m)Clear (3.2m) 0.0m (Building Over)1.5m (Within 3m)3.0m Limit4.5m (Clear) 3\. Drain Depth Below Ground (Invert)Not sure? How to measure in 2 mins 1.35m Deep Shallow (~0.9m)Typical (~1.35m)Deep (~2.2m) 0.6m Patio1.4m Typical UK2.0m Self-Cert Cap3.5m Deep 4\. Drain Pipe Diameter & Pipe Material 100mm (4" Domestic) Self-Cert OK 150mm (6" Shared) Most Common UK 225mm (9" Collector) Formal Review 300mm (12" Trunk) Formal Class 2 Existing Sewer Pipe Material: Vitreous Clay Pre-1980 (Rockers) PVCu Plastic Modern Flexible Pitch Fibre 1950s–70s Relay 5\. Three Quick Site Checks Manhole cover would end up inside the new roomRequires external patio chamber diversion (IC-2B) Shared drain with neighbour (Public Sewer under 2011 Act)Uncheck only if the pipe serves solely your own house Pressurised Pumped Rising Main (Rare)Requires mandatory 3.0m–5.0m stand-off easement Thames Water • Building Regs Part H4 Verdict ### Eligible for Fast-Track Online Self-Certification The sewer meets standard domestic criteria (150mm clay, 1.35m invert depth). You qualify for fast-track self-certification with Thames Water without submitting full engineering calculations. Water Board Fee£0.00Saves £343 Formal Fee Min. Trench Depth1.50mInvert 1.35m + 150mm Bridging Lintel Span1650mmIncludes 2 × 450mm bearings Side Cutaway (1:20)Bird's-Eye Site Plan (1:100)Show Both Diagrams Live Scale Drawing 1:20 Architectural Cutaway — Extension Wall, Foundation & Sewer Pipe Drag the blue sewer pipe left/right or up/down ①Foundation Trench Depth (Invert + 150mm)1.50m Deep Because your wall sits inside the 45° sewer load cone, your builder must excavate foundation trenches down to 1.50m (1.35m drain invert + 150mm) so building weight transfers safely into undisturbed soil below the pipe. ②Bridging Lintel & 450mm Side Bearings1650mm Span Install a 1650mm precast or cast-in-situ reinforced concrete lintel spanning across the sewer trench, resting on 450mm concrete padstone piers on solid ground either side of the pipe. ③Compressible Crown Void & Pea Shingle Bed150mm Void + 100mm Shingle Leave a strict 150mm vertical gap (filled with compressible polystyrene void board and 12mm marine ply shuttering) above the top of the Ø150mm pipe, with 100mm pea shingle gravel packed around the pipe walls. ④Flexible Rocker Joints & WRc MSCC5 CCTVMandatory CCTV Fit short flexible rocker joints within 150mm–600mm of where the pipe enters/exits the wall, and record a pre-build and post-build CCTV camera check for Building Control sign-off. Step-by-Step Action Plan #### Good News: You Qualify for Thames Water Self-Certification FREE (£0 Water Board Fee)Instant Online Approval Step 1 ##### Register Online with Thames (£0 Free) Complete the Thames Water online self-certification questionnaire before your builder pours concrete in the foundation trenches. Step 2 ##### Hand Your Builder the 1.50m Trench & Lintel Spec Dig foundations to 1.50m depth, form 450mm concrete side piers, and span the pipe with a 1650mm lintel and 150mm void board. Step 3 ##### Take Trench Photos for Building Control Sign-Off Photograph the open trench showing the 100mm pea shingle surround and 150mm void gap before concreting so Building Control issues your Completion Certificate. 2026 UK Fair-Price Groundwork Estimator #### What Should Your Builder & Water Board Actually Charge for This Drain Setup? Fair 2026 Total Trade Benchmark£465 – £750 1\. Thames Fee£0£0 Free Self-Cert 2\. WRc MSCC5 CCTV£165 – £240Pre & post-build camera log 3\. RC Lintel & Void£185 – £3201650mm lintel + 150mm board 4\. Shingle & Rockers£115 – £190100mm pea gravel + couplings 5\. Manhole Relocation£0 (None)No internal cover **Builder Quote Protection Tip:** Contractors often add **£800–£1,500** for “unforeseen drainage bridging variations” when no 1:20 lintel drawing or Bill of Quantities is attached to the quote.\+ Include Builder BoQ Sheet (+£19) #### Neighbour Law Alert: Party Wall etc. Act 1996 Section 6 Digging your trench down to **1.50m** within 3 metres of a neighbour's shallower foundations legally requires serving a 1-Month Party Wall Section 6 Notice before excavation starts. [Check 3m/6m Notice Rules on PartyWallChecker.co.uk →](https://partywallchecker.co.uk) \[ PART H4 SUBMISSION DOSSIER READY \] Need Official Drawings for Your Builder, Building Control or Thames?Instant 4-Sheet PDF Dossier (1:20 Cross-Section, Lintel Schedule, 1:100 Site Plan & CCTV Checklist) — **Saves £425+ vs Architect Fees** Preview Live Sample A4 SheetThames Water Ready Optional Engineering & Legal Add-On Bumps for Your Groundwork Add Party Wall Act s.6 (3m/6m Deep Excavation) Neighbour Notice & Consent Template RECOMMENDED FOR 1.50m TRENCH+£14.99 Statutory 1-Month deep excavation notice and 14-day formal positive assent letter with 1:20 trench section diagram under s.6(5). Add Builder Groundwork Bill of Quantities & Fixed-Price Lintel Tender Sheet STOPS BUILDER EXTRAS+£19.00 Itemised pea shingle m³, GEN3/C35 trench fill concrete volume, compressible crown board, and Class B engineering brick bearings schedule. [Download Official Part H4 CAD Pack (£39.98)](/application-pack?authority=thames-water&tier=self_cert&distance=0&depth=1.35&diameter=150&material=clay&manhole=0&mode=pre_build&pwBump=1&boqBump=0)Book Local CCTV / Civils Quote Copy Free Builder SpecShare Setup Link with Builder / Architect Free text summary: 1.50m trench • 1650mm lintel • 150mm void Thames• £0 Self-Cert Route • 1.50m Trench • 1650mm Lintel• 1.50m Trench • 1650mm Lintel Book Local CCTV Survey[Get 4-Sheet Part H4 CAD Pack (£39.98) →](/application-pack?authority=thames-water&tier=self_cert&distance=0&depth=1.35&diameter=150&material=clay&manhole=0&mode=pre_build&pwBump=1&boqBump=0) ### Structural Engineering Checklist & Lintel Detail **Recommended Solution:** Installation of recessed screw-down airtight double-seal inspection cover rated for interior foot traffic. Front inspection chambers located under driveways adjacent to garage doors Flooring insulation and screed pour encapsulating drain inspection covers Need to maintain access for high-pressure jetting and vacuum tankers UK Extension & Building Control Workflow ### Required Ancillary Services & Statutory Approvals Complete your Approved Document H4 drainage sign-off, pre-build WRc CCTV survey, and adjacent structural approvals in one place. Step 1 • Drainage Approval BuildOverAgreement.co.uk #### Instant 4-Sheet Part H4 CAD & Method Statement Pack (£24.99) Pre-computed 1:20 trench cross-section CAD, 1:100 site plan, 45° load dispersion calculations, and WRc MSCC5 CCTV surveyor brief. [Unlock Part H4 Dossier (£24.99)](/application-pack) Mandatory for Part H4 UK Drainage Surveyor Network #### Fast-Track Local Pre & Post Construction CCTV Surveyors Compare vetted WRc-coded drain camera specialists in your postcode for mandatory pre-build and post-build MSCC5 video surveys. Get Free WRc CCTV Quotes Step 2 • Party Wall Act PartyWallChecker.co.uk #### Excavating Deeper for Sewer Clearance? Check Section 6 Rules Digging foundations to 150mm below sewer invert depth within 3m of next door triggers a mandatory Section 6 Party Wall Notice. [Check 3m/6m Party Wall Trigger](https://partywallchecker.co.uk) Step 3 • Structural Steel RSJCalculator.co.uk #### Knocking Through Your Rear Wall? Size Your Eurocode 3 RSJ Beam Instant BS EN 1993-1-1 Universal Beam sizing, concrete padstone bearing calculations, and Building Control Part A calculation packs. [Calculate RSJ & Padstones](https://rsjcalculator.co.uk) Step 4 • Highway Access DroppedKerbCalculator.co.uk #### Converting Front Garden Parking? Check Council s.184 Rules Verify council 4.8m × 2.4m frontage depth rules, NRSWA streetworks licensing fees, and vetted local crossover contractors. [Check Section 184 Approval](https://droppedkerbcalculator.co.uk) Step 5 • Site Utilities BLUETTI UK Official Partner #### Clean Site Power for Submersible Trench Pumps & Outbuildings Zero-emission LiFePO4 battery stations for dewatering trench pumps, CCTV rigs, and off-grid garden room power without Part P trenching. [View Site Power & Solar Kits](https://www.awin1.com/cread.php?awinmid=32273&awinaffid=3035391&clickref=navre_offerwall_power&ued=https%3A%2F%2Fwww.bluettipower.co.uk) UK Extension & Structural Compliance Suite: [PartyWallChecker.co.uk](https://partywallchecker.co.uk "Party Wall Act 1996 Notice Checker")[RSJCalculator.co.uk](https://rsjcalculator.co.uk "BS EN 1993-1-1 RSJ Steel Beam & Padstone Sizer")[SoakawayCalculator.co.uk](https://soakawaycalculator.co.uk "BRE Digest 365 Extension Soakaway Sizing")[DroppedKerbCalculator.co.uk](https://droppedkerbcalculator.co.uk "Section 184 Dropped Kerb Costs")[GardenOfficePower.co.uk](https://gardenofficepower.co.uk "BS 7671 SWA Armour Cable Calculator")[HedgeHeight.co.uk](https://hedgeheight.co.uk "BRE Part 8 Action Hedge Height Calculator") ## 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