Drainage for London Home Extensions: Regulations, SuDS & Costs

APPROVED DOCUMENT H OF THE BUILDING REGULATIONS SETS OUT THE MINIMUM STANDARDS FOR DRAINAGE AND WASTE DISPOSAL IN ENGLAND. FOR HOME EXTENSIONS, THE MOST RELEVANT SECTIONS ARE:

H1 — FOUL WATER DRAINAGE: NEW CONNECTIONS TO THE FOUL SEWER MUST BE MADE BY COMPETENT CONTRACTORS USING APPROVED MATERIALS AND METHODS. POLYPIPE 110MM PUSHFIT UNDERGROUND DRAINAGE PIPE (ORANGE) IS THE STANDARD FOR UNDERGROUND RUNS IN THE UK. MINIMUM GRADIENTS ARE 1:80 (1.25%) FOR A SINGLE WC CONNECTION AND 1:40 (2.5%) FOR MULTIPLE CONNECTIONS. INSPECTION CHAMBERS ARE REQUIRED AT EVERY CHANGE OF DIRECTION AND AT JUNCTIONS OF DRAINS, WITH MAXIMUM SPACING OF 45 METRES ON A STRAIGHT RUN.

H3 — RAINWATER DRAINAGE: SURFACE WATER FROM EXTENSIONS MUST BE DRAINED EFFECTIVELY. THE PREFERRED HIERARCHY UNDER PLANNING POLICY AND PART H3 IS: (1) SOAKAWAY; (2) WATERCOURSE; (3) SURFACE WATER SEWER; (4) COMBINED SEWER. CONNECTING DIRECTLY TO THE COMBINED SEWER IS THE LAST RESORT AND INCREASINGLY RESTRICTED IN LONDON BOROUGHS WITH OVERLOADED SEWERS.

H4 — DRAINAGE BELOW GROUND: CONCRETE DRAINS, CLAY DRAINS, AND ALL UNDERGROUND DRAINAGE MUST BE LAID TO CORRECT GRADIENTS, BEDDED IN PEA GRAVEL OR CONCRETE ACCORDING TO DEPTH AND GROUND CONDITIONS, AND INSPECTED BY BUILDING CONTROL BEFORE BACKFILLING. CCTV DRAINAGE SURVEYS BY SPECIALIST CONTRACTORS ARE STRONGLY RECOMMENDED BEFORE ANY EXTENSION GROUNDWORK TO IDENTIFY PRE-EXISTING DEFECTS IN EXISTING DRAINS THAT COULD CAUSE LIABILITY DISPUTES AFTER WORKS.

BUILDING CONTROL INVOLVEMENT: ALL DRAINAGE WORK IN CONNECTION WITH A HOME EXTENSION REQUIRES BUILDING REGULATIONS APPROVAL. YOUR BUILDING CONTROL OFFICER WILL TYPICALLY INSPECT OPEN TRENCHES BEFORE BACKFILLING — A NOTIFICATION REQUIREMENT THAT MUST BE BUILT INTO YOUR CONTRACTOR'S PROGRAMME.

THAMES WATER IS THE SEWERAGE UNDERTAKER FOR MOST OF LONDON. THEIR BUILD-OVER POLICY GOVERNS WHAT HOMEOWNERS CAN AND CANNOT DO NEAR THAMES WATER ASSETS.

KEY RULE: IF YOUR PROPOSED EXTENSION IS WITHIN 3 METRES OF A PUBLIC SEWER (A SEWER ADOPTED BY AND MAINTAINED BY THAMES WATER), YOU MUST CONTACT THAMES WATER AND OBTAIN THEIR AGREEMENT BEFORE STARTING WORK. THIS IS A STATUTORY REQUIREMENT UNDER THE WATER INDUSTRY ACT 1991.

BUILD-OVER AGREEMENT: IF YOUR EXTENSION WOULD COVER OR BE BUILT DIRECTLY OVER A PUBLIC SEWER, YOU NEED A BUILD-OVER AGREEMENT FROM THAMES WATER. THAMES WATER WILL: — REVIEW YOUR STRUCTURAL DRAWINGS — ASSESS THE IMPACT ON THE SEWER — SPECIFY BRIDGE CONSTRUCTION OR PROTECTIVE MEASURES — SET CONDITIONS FOR FOUNDATION DESIGN TO AVOID LOADING THE SEWER — REQUIRE CCTV SURVEY OF THE SEWER BEFORE AND AFTER WORKS

TIMESCALE: BUILD-OVER AGREEMENTS TYPICALLY TAKE 4-8 WEEKS BUT CAN TAKE LONGER FOR COMPLEX CASES. THAMES WATER CHARGES A FEE: £278-£350+ DEPENDING ON THE AGREEMENT TYPE.

NEAR-SEWER WORKS (WITHIN 3 METRES, NOT DIRECTLY OVER): A BUILD-OVER AGREEMENT IS STILL REQUIRED, THOUGH CONDITIONS ARE TYPICALLY LESS ONEROUS THAN A DIRECT BUILD-OVER. THAMES WATER WILL STILL REQUIRE A CCTV DRAINAGE SURVEY BEFORE AND AFTER WORKS, AND MAY SPECIFY MINIMUM FOUNDATION DEPTHS.

FINDING SEWER LOCATIONS: THAMES WATER PROVIDES FREE ONLINE SEWER MAPS VIA THEIR DEVELOPMENT SERVICES PORTAL (PROPERTY.THAMESWATER.CO.UK). ALWAYS CHECK BEFORE DESIGNING FOUNDATIONS — SEWER POSITIONS ON MAPS ARE APPROXIMATE, AND A CCTV SURVEY OR DRAINAGE CONTRACTOR TRACE IS ADVISABLE BEFORE FINALISING STRUCTURAL DESIGN.

CRITICAL RISK: BUILDING OVER A PUBLIC SEWER WITHOUT A BUILD-OVER AGREEMENT CAN RESULT IN THAMES WATER REQUIRING YOU TO DEMOLISH THE EXTENSION AT YOUR OWN COST. THIS RISK IS UNDERWRITTEN BY YOUR BUILDING INSURER IN SOME POLICIES — CHECK BEFORE STARTING WORK.

THE TOWN AND COUNTRY PLANNING (PERMITTED DEVELOPMENT) (ENGLAND) AMENDMENT ORDER 2008 MADE SUSTAINABLE DRAINAGE A REQUIREMENT FOR NEW OR REPLACEMENT FRONT GARDEN HARDSTANDING EXCEEDING 5 SQUARE METRES. THIS IS NOW A WELL-ESTABLISHED RULE THAT MANY HOMEOWNERS STILL OVERLOOK.

THE RULE: FRONT GARDEN HARDSTANDING THAT COVERS MORE THAN 5M2 AND IS NOT PERMEABLE REQUIRES PLANNING PERMISSION. PERMEABLE SURFACES — OR SURFACES THAT DRAIN TO A PERMEABLE AREA WITHIN THE CURTILAGE (NOT TO A DRAIN OR WATERCOURSE) — CAN BE INSTALLED WITHOUT PLANNING PERMISSION AS PD.

APPROVED PERMEABLE SURFACES FOR SUDS COMPLIANCE: — RESIN-BOUND AGGREGATE: RONACRETE RONASURF (£80-£110/M2 INSTALLED); ADDAGRIP TERRAWAY (£75-£100/M2); SURESET (£90-£120/M2). ALL USE A SUB-BASE OF MOT TYPE 1 150MM COMPACTED + GEOTEXTILE + 6MM CLEAN SHINGLE DRAINAGE LAYER + RESIN-BOUND SURFACE 18MM. — BLOCK PAVING WITH PERMEABLE JOINTING: MARSHALLS PRIORA PERMEABLE BLOCK PAVING (£60-£90/M2) WITH SINGLE-SIZE AGGREGATE IN JOINTS — LAID ON GEOTEXTILE + SINGLE-SIZE AGGREGATE BED INSTEAD OF SHARP SAND. — GRAVEL/SHINGLE: NATURAL STONE SHINGLE 20MM ON GEOTEXTILE IS INHERENTLY PERMEABLE. — GRASS/GRAVEL SYSTEMS: GRASS PAVERS OR GRASS-FILLED GRID SYSTEMS (E.G. BODPAVE 85) PROVIDE PERMEABLE HARD STANDING FOR OCCASIONAL USE.

SUDS FOR EXTENSIONS: FOR REAR EXTENSIONS, NEW PATIO AREAS ARE NOT RESTRICTED BY THE 2008 ORDER (WHICH ONLY APPLIES TO FRONT GARDENS). HOWEVER, BUILDING REGULATIONS H3 STILL REQUIRES EFFECTIVE SURFACE WATER DISPOSAL. A PERIMETER CHANNEL DRAIN (MARSHAL CHANNEL DRAIN 100MM, RECYFIX OR ACO THRESHOLD DRAIN) CONNECTING TO A SOAKAWAY IS THE PREFERRED SOLUTION WHERE GROUND CONDITIONS ALLOW. SOAKAWAY SIZE IS CALCULATED BASED ON THE DRAINED AREA AND SOIL PERMEABILITY — TYPICALLY A 1.5M3 STONE-FILLED CRATE SOAKAWAY (WAVIN GEOCELLAR OR POLYSTORM CELL) FOR A 20-30M2 PATIO.

SUDS IN PLANNING CONDITIONS: MANY LONDON BOROUGHS NOW APPLY A SuDS CONDITION TO PLANNING CONSENTS FOR EXTENSIONS, REQUIRING A DRAINAGE STRATEGY TO BE SUBMITTED AND APPROVED BEFORE GROUNDWORK STARTS. THIS STRATEGY TYPICALLY REQUIRES A PERCOLATION TEST (BS 6297:2007) TO ASSESS SOAKAWAY FEASIBILITY IN LONDON CLAY.

LONDON CLAY COVERS A LARGE PROPORTION OF INNER AND OUTER LONDON. ITS LOW PERMEABILITY — TYPICALLY 10-8 TO 10-9 M/S — MEANS THAT SOAKAWAYS OFTEN CANNOT BE USED IN CLAY-DOMINANT AREAS. THIS HAS SIGNIFICANT IMPLICATIONS FOR DRAINAGE DESIGN.

PERCOLATION TEST (BS 6297:2007): BEFORE DESIGNING A SOAKAWAY, A PERCOLATION TEST MUST BE CARRIED OUT TO DETERMINE WHETHER THE GROUND CAN ACCEPT SURFACE WATER AT A SUFFICIENT RATE. THE TEST INVOLVES DIGGING A PIT 300MM SQUARE AND 300MM DEEP, FILLING IT WITH WATER AND TIMING THE DRAINAGE. IF THE SOIL ABSORPTION RATE IS BELOW 12 SECONDS PER MILLIMETRE, A SOAKAWAY IS NOT VIABLE.

IN LONDON CLAY: PERCOLATION TESTS IN LONDON CLAY TYPICALLY GIVE RATES OF 100+ SECONDS/MM — FAR TOO SLOW FOR A CONVENTIONAL SOAKAWAY. THE ALTERNATIVES ARE: — CONNECT TO A SURFACE WATER SEWER: REQUIRES THAMES WATER APPROVAL AND AN APPROPRIATELY SIZED RESTRICTED-FLOW CONNECTION (FLOW RESTRICTION DEVICES SUCH AS HYDRO-BRAKES OR VORTEX FLOW CONTROLS ARE OFTEN REQUIRED TO LIMIT DISCHARGE RATE). — CONNECT TO A COMBINED SEWER: PERMITTED AS LAST RESORT IN THE DRAINAGE HIERARCHY BUT THAMES WATER INCREASINGLY REQUIRES FLOW ATTENUATION. — ATTENUATION WITH CONTROLLED RELEASE: A BELOW-GROUND STORAGE TANK OR GEOCELLULAR CRATES (ATLANTIS POLYSTORM, WAVIN GEOCELLAR) THAT STORE RUNOFF AND RELEASE AT A CONTROLLED RATE TO THE SEWER. THIS IS NOW THE STANDARD SOLUTION FOR MANY LONDON EXTENSIONS.

FOUNDATIONS IN LONDON CLAY: LONDON CLAY IS A SHRINK-SWELL SOIL. SEASONAL MOISTURE CHANGES AND TREE ROOT ACTIVITY CAUSE VOLUMETRIC CHANGES THAT CAN DAMAGE SHALLOW FOUNDATIONS. RAFT FOUNDATIONS OR DEEP STRIP FOUNDATIONS (1.5-2.5M IN HEAVILY TREED LONDON GARDENS) ARE OFTEN REQUIRED. NHBC TECHNICAL STANDARDS AND BUILDING CONTROL GUIDANCE SHOULD BE FOLLOWED. ALWAYS ENGAGE A STRUCTURAL ENGINEER EXPERIENCED IN LONDON CLAY CONDITIONS.

PRACTICAL DRAINAGE COSTS FOR A TYPICAL LONDON SINGLE-STOREY REAR EXTENSION WITH KITCHEN-DINER:

DRAINAGE DESIGN AND CALCULATIONS: £500-£1,500 (PART OF STRUCTURAL ENGINEER'S OR DRAINAGE SPECIALIST'S SCOPE)

CCTV DRAINAGE SURVEY (PRE-WORKS): £250-£450 (ESSENTIAL — ESTABLISHES PRE-EXISTING CONDITION)

THAMES WATER BUILD-OVER AGREEMENT (IF NEEDED): £278-£500 PLUS CONTRACTOR TIME TO PREPARE DOCUMENTATION

FOUL WATER CONNECTION TO EXISTING SOIL STACK OR DRAIN: £500-£1,500 (DEPENDING ON DISTANCE AND ACCESS)

NEW DRAINAGE RUN — UNDERGROUND 110MM POLYPIPE WITH INSPECTION CHAMBERS: £150-£300/LINEAR METRE INSTALLED (MORE IN DEEP TRENCHES OR ROCKY GROUND)

ATTENUATION TANK WITH CONTROLLED RELEASE (WHERE SOAKAWAY NOT VIABLE): £1,500-£4,000 (GEOCELLULAR CRATES + FLOW CONTROL DEVICE + INSTALLATION)

PERMEABLE PATIO SURFACE (RESIN BOUND): £80-£110/M2 (25M2 PATIO = £2,000-£2,750)

SURFACE WATER CHANNEL DRAIN WITH GULLY: £300-£600 PER LINEAR RUN

CCTV DRAINAGE SURVEY (POST-WORKS, FOR THAMES WATER): £250-£450

TOTAL DRAINAGE BUDGET FOR A TYPICAL LONDON REAR EXTENSION: £3,000-£8,000 DEPENDING ON CONNECTIVITY, THAMES WATER PROXIMITY, AND HARDSTANDING AREA.

CONTRACTOR TIP: ALWAYS ALLOW A DRAINAGE CONTINGENCY IN YOUR EXTENSION BUDGET. DISCOVERING THAT AN EXISTING DRAIN IS CRACKED, OFFSET, OR ROOT-INVADED (ALL COMMON IN LONDON'S VICTORIAN-ERA DRAINAGE INFRASTRUCTURE) DURING A PRE-WORKS CCTV SURVEY GIVES YOU THE OPPORTUNITY TO BUDGET CORRECTLY — RATHER THAN FINDING THE SAME ISSUE MID-BUILD WHEN EXCAVATION IS OPEN AND PROGRAMME PRESSURE IS HIGH.