Drainage Design for London Home Extensions: Underground Drainage, SuDS, Sewer Build-Over, and CCTV Survey Guide

THE FIRST STEP IN ANY LONDON EXTENSION DRAINAGE DESIGN IS ESTABLISHING THE POSITION, DEPTH, CONDITION, AND OWNERSHIP OF ALL DRAINS AND SEWERS BENEATH AND ADJACENT TO THE EXTENSION FOOTPRINT.

THAMES WATER SEWER RECORDS: THAMES WATER (THE SEWERAGE UNDERTAKER FOR MOST OF LONDON) PROVIDES RECORDS OF PUBLIC SEWERS VIA THEIR LIVE INTERACTIVE MAPPING SYSTEM (THAMES WATER DRAINAGE RECORDS — ACCESSIBLE VIA GOV.UK PLANNING PORTAL, OR BY DIRECT APPLICATION TO THAMES WATER'S CONNECTIONS TEAM). THE MAPPING SHOWS THE APPROXIMATE POSITION OF PUBLIC SEWERS (PUBLIC SEWERS BEING THOSE ADOPTED BY THAMES WATER FROM 1 OCTOBER 2011 UNDER THE WATER INDUSTRY ACT 1991 SECTION 24 — WHICH INCLUDED ALL PREVIOUSLY PRIVATE SHARED LATERAL DRAINS). NOTE: BEFORE 2011, SHARED DRAINS BETWEEN PROPERTIES WERE PRIVATE (EACH PROPERTY OWNER'S RESPONSIBILITY) — SINCE OCTOBER 2011, ALL SHARED DRAINS SERVING MORE THAN ONE PROPERTY HAVE BEEN ADOPTED AS PUBLIC SEWERS. THIS IS A CRITICAL DISTINCTION FOR LONDON RENOVATION WHERE THE SHARED DRAIN BETWEEN TWO PROPERTIES (COMMONLY RUNNING THROUGH GARDENS) IS NOW A PUBLIC SEWER SUBJECT TO BUILD-OVER REGULATIONS.

CCTV DRAIN SURVEY: A CCTV DRAIN SURVEY (CLOSED-CIRCUIT TELEVISION DRAIN INSPECTION) INVOLVES INSERTING A CAMERA INTO THE DRAIN AND REMOTELY INSPECTING THE INTERNAL CONDITION. CCTV SURVEYS REVEAL: PIPE CONDITION (CRACKED, FRACTURED, OFFSET, DEFORMED), ROOT INGRESS (FROM LONDON PLANE TREES, WILLOWS, OAKS — COMMON IN LONDON GARDEN DRAINS), SILT DEPOSITS, INCORRECT GRADIENTS, COLLAPSED PIPES, AND THE POSITION OF JUNCTIONS. COST: £250 TO £500 FOR A STANDARD RESIDENTIAL CCTV SURVEY OF A SINGLE PROPERTY'S DRAIN RUNS. FOR PRE-CONTRACT SURVEYS AHEAD OF A REAR EXTENSION, A CCTV SURVEY IS STRONGLY RECOMMENDED — ESPECIALLY WHERE GROUNDWORKS ARE PLANNED WITHIN 3.0M OF ANY DRAIN RUN.

A THAMES WATER BUILD-OVER AGREEMENT IS REQUIRED BEFORE ANY BUILDING WORK CAN BE CONSTRUCTED OVER OR WITHIN 3.0M OF A PUBLIC SEWER OR LATERAL DRAIN.

CRITERIA REQUIRING FORMAL BUILD-OVER APPLICATION: BUILDING OVER A PUBLIC SEWER (THE EXTENSION FOOTPRINT CROSSES THE CENTRELINE OF A PUBLIC SEWER). BUILDING WITHIN 3.0M OF A PUBLIC SEWER (THE EXTENSION FOUNDATIONS WILL BE WITHIN 3.0M OF THE NEAREST EDGE OF THE PUBLIC SEWER). INTRODUCING NEW DRAINAGE CONNECTIONS TO A PUBLIC SEWER (A NEW INSPECTION CHAMBER OR DRAIN CONNECTING INTO AN EXISTING PUBLIC SEWER).

BUILD-OVER AGREEMENT CONDITIONS: THAMES WATER WILL APPROVE A BUILD-OVER AGREEMENT SUBJECT TO STANDARD CONDITIONS INCLUDING: MINIMUM 100MM CLEARANCE BETWEEN THE UNDERSIDE OF THE FOUNDATION AND THE TOP OF THE SEWER PIPE. FLEXIBLE JOINTS IN THE SEWER PIPE WITHIN THE BUILDING FOOTPRINT (TO ALLOW DIFFERENTIAL SETTLEMENT WITHOUT PIPE FRACTURE). INSPECTION CHAMBERS WITHIN 1.0M OF THE BUILDING FACE TO ALLOW FUTURE ACCESS. NO PERMANENT STRUCTURE WITHIN 1.0M OF THE SEWER CENTRELINE (UNLESS STRUCTURAL ENGINEER DESIGN IS SUBMITTED). PRE- AND POST-CONSTRUCTION CCTV SURVEYS OF THE SEWER (THAMES WATER REQUIRE THESE TO PROTECT AGAINST CLAIMS FOR DAMAGE CAUSED BY THE BUILDING WORKS). THE BUILD-OVER AGREEMENT APPLICATION IS MADE VIA THAMES WATER'S ONLINE PORTAL. THAMES WATER'S DECISION TIMESCALE: TYPICALLY 2 TO 4 MONTHS — WHICH MUST BE FACTORED INTO THE EXTENSION PROGRAMME. APPLICATIONS UNDER THE "SELF-CERTIFICATION" ROUTE (WHERE THE SEWER IS LESS THAN 160MM DIAMETER, CIRCULAR, AND AT ADEQUATE DEPTH) CAN BE FASTER.

COST OF BUILD-OVER AGREEMENT: THAMES WATER'S FEE FOR A BUILD-OVER AGREEMENT (STANDARD APPLICATION): APPROXIMATELY £300 TO £500 DEPENDING ON SEWER SIZE AND COMPLEXITY. PLUS CCTV SURVEY COSTS (PRE AND POST): £500 TO £1,000 TOTAL.

FOUL WATER DRAINAGE FROM LONDON EXTENSIONS MUST BE DESIGNED TO BUILDING REGULATIONS PART H (DRAINAGE AND WASTE DISPOSAL) STANDARDS.

GRADIENTS FOR FOUL DRAINS: BUILDING REGULATIONS PART H REQUIRES ADEQUATE SELF-CLEANING VELOCITY IN FOUL DRAINS TO PREVENT BLOCKAGES. MINIMUM GRADIENTS FOR POLYPIPE (OR EQUIVALENT) UNDERGROUND DRAINAGE SYSTEMS: 100MM DIAMETER DRAIN: MINIMUM 1 IN 40 GRADIENT (1:40 = 25MM FALL PER METRE). 150MM DIAMETER DRAIN: MINIMUM 1 IN 150 GRADIENT (1:150 = 6.7MM FALL PER METRE). FOR LONDON EXTENSIONS WHERE THE GROUND IS OFTEN VERY FLAT AND THE EXISTING DRAIN CONNECTION POINT IS NOT DEEP, ACHIEVING ADEQUATE GRADIENT CAN BE CHALLENGING — EARLY SURVEY OF DRAIN INVERT LEVELS IS IMPORTANT.

POLYPIPE UNDERGROUND DRAINAGE: POLYPIPE 110MM (NOMINAL 100MM) PUSHFIT UNDERGROUND DRAINAGE IN ORANGE MDPE IS THE STANDARD FOR LONDON RESIDENTIAL FOUL DRAINAGE. PIPES ARE BEDDED IN 100MM GRANULAR MATERIAL (10MM SINGLE-SIZE GRAVEL OR CLEAN CRUSHED STONE) BELOW AND MINIMUM 150MM COMPACTED GRANULAR SURROUND TO CROWN OF PIPE, THEN SELECTED FILL. ACO DRAIN LTD AND POLYPIPE UNDERGROUND PRODUCE A COMPREHENSIVE RANGE OF INSPECTION CHAMBERS, BENDS, JUNCTIONS, AND ACCESS FITTINGS FOR RESIDENTIAL DRAINAGE INSTALLATION.

INSPECTION CHAMBERS (ICS): PART H REQUIRES RODDING POINTS OR INSPECTION CHAMBERS AT ALL BENDS, JUNCTIONS, AND AT MAXIMUM 45M INTERVALS ON STRAIGHT RUNS. POLYPIPE 315MM DIAMETER PRECAST POLYPROPYLENE INSPECTION CHAMBER (WITH DOUBLE-SEALED ACCESS COVER IN D400 CLASS LOADING FOR DRIVEWAY LOCATIONS OR A15 LOADING FOR GARDEN LOCATIONS) IS THE STANDARD SPECIFICATION. FOR DEEPER MANHOLES (INVERT MORE THAN 1.2M), PRECAST CONCRETE RINGS (1050MM DIAMETER OR LARGER) ARE USED PER BS 5911.

SURFACE WATER DRAINAGE FROM LONDON HOME EXTENSIONS AND NEW PAVED AREAS IS SUBJECT TO BUILDING REGULATIONS PART H AND, INCREASINGLY, MANDATORY SUSTAINABLE DRAINAGE SYSTEM (SUDS) REQUIREMENTS.

THE SUDS HIERARCHY (PART H AND LONDON PLAN POLICY SI 13): SURFACE WATER FROM NEW DEVELOPMENT MUST BE MANAGED IN THE FOLLOWING PRIORITY ORDER (HIGHEST TO LOWEST): 1. DISCHARGED TO A SOAK — WHERE GROUND CONDITIONS ALLOW (SOAKAWAY OR INFILTRATION TRENCH). 2. DISCHARGED TO A WATERCOURSE (DITCH, STREAM) — WHERE AVAILABLE AND PERMITTED. 3. ATTENUATION AND RESTRICTED DISCHARGE TO THE PUBLIC SEWER — WHERE INFILTRATION AND WATERCOURSE OPTIONS ARE NOT AVAILABLE. IN MOST OF LONDON (LONDON CLAY GROUND — VERY LOW PERMEABILITY), DIRECT SOAKAWAYS DO NOT WORK — LONDON CLAY WILL NOT DRAIN AT SUFFICIENT RATE. HOWEVER, POLYPIPE STORMCELL ATTENUATION CRATES BURIED IN A GRAVEL-WRAPPED PERMEABLE CELL CAN PROVIDE TEMPORARY STORAGE OF STORMWATER (ATTENUATION), RELEASING IT SLOWLY TO A DITCH OR RESTRICTED CONNECTION TO THE SEWER.

POLYPIPE STORMCELL (UNDERGROUND ATTENUATION CRATE SYSTEM): STORMCELL IS A MODULAR POLYPROPYLENE CRATE SYSTEM THAT CREATES AN UNDERGROUND VOID VOLUME FOR SURFACE WATER STORAGE. THE CRATES ARE WRAPPED IN PERMEABLE GEOTEXTILE AND BURIED IN EXCAVATION — INSTALLED BELOW GARDEN LEVEL. POLYPIPE STORMCELL ATTENUATION: STORAGE VOLUME 95% VOID RATIO PER CRATE. STANDARD CRATE 800MM (L) X 400MM (W) X 400MM (H) = 0.128 M³ STORAGE PER CRATE. FOR A TYPICAL LONDON EXTENSION ROOF AREA OF 25 M² IN A 1:100 YEAR RAIN EVENT (LONDON DESIGN STORM 40 MM IN 1 HOUR — BUILDING REGULATIONS): STORMWATER VOLUME APPROXIMATELY 1.0 M³ — REQUIRING APPROXIMATELY 8 POLYPIPE STORMCELL CRATES FOR 1 HOUR ATTENUATION.

ACO SURFACE WATER CHANNEL DRAINS: ACO Q-DRAIN AND ACO K-DRAIN POLYMER CONCRETE CHANNEL DRAINS ARE WIDELY USED IN LONDON EXTENSION THRESHOLD DRAINAGE (AT THE BIFOLD OR SLIDING DOOR THRESHOLD TO INTERCEPT SURFACE WATER RUNOFF FROM THE EXTENSION ROOF BEFORE IT ENTERS THE BUILDING) AND IN GARDEN PATIO DRAINAGE. ACO DRAIN SIZES: 100MM INTERNAL WIDTH (Q100) FOR RESIDENTIAL GARDEN PATIO USE, CONNECTED TO 110MM POLYPIPE UNDERGROUND DRAINAGE.

LOFT CONVERSIONS IN LONDON THAT ADD A BATHROOM OR EN SUITE REQUIRE THE SOIL STACK (THE MAIN VERTICAL DRAIN PIPE CARRYING SOIL WASTE FROM WC AND FOUL DRAINAGE FROM BATHS/SHOWERS) TO BE EXTENDED TO SERVE THE NEW BATHROOM.

SOIL STACK EXTENSION FOR LOFT BATHROOM: THE EXISTING 110MM MDPE OR PVC SOIL STACK (TYPICALLY RUN INTERNALLY THROUGH THE HOUSE ON MULTI-STOREY PROPERTIES) MUST BE EXTENDED VERTICALLY TO SERVE THE NEW LOFT BATHROOM WC, BASIN, AND SHOWER. WHERE THE STACK PASSES THROUGH WALLS OR FLOORS IN THE LOFT, PIPE BOXING (TYPICALLY MDF OR TIMBER FRAMED WITH PLYWOOD FACE) IS USED TO CONCEAL THE STACK WHILE MAINTAINING ACCESS AT SERVICE INTERVALS. THE TOP OF THE SOIL STACK MUST VENT TO ATMOSPHERE (OPEN VENT 900MM ABOVE ANY OPENING WINDOW IN THE EXTERNAL WALL) OR TERMINATE IN A DURGO AIR ADMITTANCE VALVE (AAV) — SUREVENT OR HEPWORTH DURGO — WHERE AN OPEN VENT IS NOT POSSIBLE (E.G., CONSERVATION AREA WHERE EXTERNAL PIPES ARE RESTRICTED).

RAINWATER DRAINAGE FOR LOFT DORMER: THE FLAT ROOF OF A REAR DORMER LOFT CONVERSION REQUIRES A RAINWATER DRAINAGE SYSTEM — TYPICALLY A 75MM OR 100MM RAINWATER OUTLET THROUGH THE FLAT ROOF DECK CONNECTED TO A RAINWATER DOWNPIPE (68MM OR 110MM PVC). THE DOWNPIPE CONNECTS AT THE BASE TO A BACK INLET GULLY AND THEN TO THE SURFACE WATER OR COMBINED DRAIN.

DRAINAGE COSTS IN LONDON VARY CONSIDERABLY BASED ON THE DEPTH OF EXISTING DRAINS, THE PRESENCE OF SEWER BUILD-OVER REQUIREMENTS, AND THE SURFACE WATER DRAINAGE STRATEGY.

SEWER BUILD-OVER AGREEMENT (WHERE REQUIRED): THAMES WATER FEE: £300 TO £500. CCTV SURVEY (PRE AND POST): £500 TO £1,000. STRUCTURAL ENGINEER FEE (IF SPECIAL DESIGN REQUIRED): £500 TO £1,500. TOTAL BUILD-OVER COST: £1,300 TO £3,000 AS A SEPARATE PRECONSTRUCTION COST.

FOUL WATER DRAINAGE FOR STANDARD LONDON REAR EXTENSION (NEW INSPECTION CHAMBER, 4M POLYPIPE 110MM DRAIN, CONNECTION TO EXISTING DRAIN OR SEWER, EXCAVATION AND BACKFILL): £800 TO £2,500 DEPENDING ON DEPTH AND ACCESS.

SURFACE WATER DRAINAGE (INCLUDING 3 POLYPIPE STORMCELL ATTENUATION CRATES + ACO Q100 CHANNEL DRAIN AT BIFOLD THRESHOLD + 6M POLYPIPE SURFACE WATER DRAIN): £1,500 TO £3,500 DEPENDING ON SITE COMPLEXITY.

DRAINAGE CCTV SURVEY (PRE-CONTRACT): £250 TO £500 — ONE OF THE BEST-VALUE PRE-CONSTRUCTION INVESTMENTS FOR ANY LONDON EXTENSION PROJECT.