Basement Lightwells for London Victorian Terraces: Construction, Waterproofing, and Planning

THERE ARE THREE PRINCIPAL SCENARIOS FOR BASEMENT LIGHTWELLS IN LONDON VICTORIAN TERRACE PROPERTIES:

SCENARIO 1 — EXISTING AREA (LIGHTWELL) ALREADY PRESENT: MANY LONDON VICTORIAN TERRACES (PARTICULARLY THOSE WITH A LOWER-GROUND-FLOOR LEVEL — COMMON IN LARGER SEMI-DETACHED AND END-OF-TERRACE PROPERTIES IN INNER LONDON) HAVE AN EXISTING BRICK-LINED FRONT AREA (AREAWAY) — TYPICALLY 800MM TO 1500MM DEEP, 1200-1800MM WIDE, AND 900-1800MM IN LENGTH BELOW THE FRONT GARDEN LEVEL. THESE EXISTING AREAS MAY NEED: WATERPROOFING (IF THE BRICK LINING IS DAMP-PENETRATED OR THE EXISTING BITUMINOUS TANKING HAS FAILED). DEEPENING (IF THE EXISTING AREA IS TOO SHALLOW TO PROVIDE ADEQUATE DAYLIGHTING TO A NEWLY HABITABLE BASEMENT ROOM — THE SILL OF THE NEW WINDOW SHOULD BE AT OR BELOW EYE LEVEL FROM STANDING IN THE LIGHTWELL). DRAINAGE UPGRADING (IF THE EXISTING AREA DRAIN IS BLOCKED, UNDERSIZED, OR CONNECTED TO THE SURFACE WATER DRAINAGE SYSTEM IN A WAY THAT CAUSES FLOODING).

SCENARIO 2 — NO EXISTING LIGHTWELL (BASEMENT BELOW GARDEN LEVEL WITHOUT A LIGHTWELL): SOME LONDON VICTORIAN TERRACES HAVE A BASEMENT LEVEL BELOW THE FRONT GARDEN WITHOUT ANY EXISTING LIGHTWELL — THE BASEMENT WAS USED AS A COAL STORE, BOILER ROOM, OR UTILITY SPACE AND HAS NO NATURAL LIGHT OR VENTILATION. FOR THIS SCENARIO: A NEW LIGHTWELL MUST BE EXCAVATED IN THE FRONT GARDEN (BELOW EXISTING GARDEN LEVEL) OR THE REAR GARDEN (FOR A REAR BASEMENT LIGHTWELL — LESS COMMON AS MOST LONDON VICTORIAN TERRACE BASEMENTS FACE THE FRONT STREET). A NEW LIGHTWELL IN THE FRONT GARDEN OF A LONDON VICTORIAN TERRACE BASEMENT IS A SIGNIFICANT STRUCTURAL UNDERTAKING: UNDERPINNING OR CANTILEVERING THE FRONT GARDEN WALL ABOVE THE LIGHTWELL, EXCAVATION IN LONDON CLAY TO THE REQUIRED DEPTH, AND WATERPROOFING OF THE NEW LIGHTWELL WALLS.

SCENARIO 3 — DEEPENING AN EXISTING AREA (LIGHTWELL BELOW EXISTING LEVEL TO GAIN HEAD HEIGHT): WHERE A LONDON VICTORIAN TERRACE BASEMENT IS BEING CONVERTED TO HABITABLE USE BUT THE EXISTING FLOOR-TO-CEILING HEIGHT IS INSUFFICIENT (LESS THAN 2100MM MINIMUM — 2300MM PREFERRED), THE SOLUTION IS OFTEN TO LOWER THE BASEMENT FLOOR LEVEL (BY EXCAVATING DOWN IN LONDON CLAY — TYPICALLY 200-500MM ADDITIONAL DEPTH) — WHICH ALSO REQUIRES DEEPENING THE EXISTING FRONT AREA/LIGHTWELL TO MAINTAIN THE DAYLIGHTING RELATIONSHIP BETWEEN THE NEW LOWER WINDOW POSITION AND THE LIGHTWELL FLOOR.

BASEMENT LIGHTWELL EXCAVATION IN LONDON REQUIRES SPECIALIST KNOWLEDGE OF LONDON CLAY — ONE OF THE MOST CHALLENGING SUBSOILS FOR DEEP EXCAVATION IN URBAN ENVIRONMENTS.

LONDON CLAY PROPERTIES RELEVANT TO LIGHTWELL EXCAVATION: BEARING CAPACITY: LONDON CLAY IS A STIFF FISSURED OVER-CONSOLIDATED CLAY — BEARING CAPACITY IS GENERALLY ADEQUATE FOR LIGHTLY LOADED RESIDENTIAL STRUCTURES BUT REQUIRES CAREFUL ASSESSMENT WHERE EXCAVATION IS DEEP OR ADJACENT TO EXISTING FOUNDATIONS. SHRINKAGE AND SWELLING: LONDON CLAY IS HIGHLY EXPANSIVE — IT SHRINKS SIGNIFICANTLY DURING DROUGHT CONDITIONS (EXACERBATED BY TREE ROOT UPTAKE — LONDON PLANE AND POPLAR TREES IN LONDON FRONT GARDENS EXTEND ROOT ZONES 5-15M) AND SWELLS ON REWETTING. EXCAVATIONS IN LONDON CLAY ADJACENT TO EXISTING TERRACE FOUNDATIONS MUST BE ASSESSED BY A STRUCTURAL ENGINEER — PARTY WALL ACT SECTION 6 (EXCAVATION WITHIN 3M OF A NEIGHBOURING FOUNDATION WHERE THE EXCAVATION IS DEEPER THAN THE NEIGHBOURING FOUNDATION) APPLIES TO MOST LONDON VICTORIAN TERRACE LIGHTWELL DEEPENING PROJECTS.

NEW LIGHTWELL STRUCTURAL OPTIONS: REINFORCED CONCRETE RETAINING WALLS (THE MOST COMMON STRUCTURAL SOLUTION FOR A NEW LIGHTWELL IN A LONDON VICTORIAN TERRACE): 150-200MM THICK RC WALLS (C30/37 CONCRETE, A393 REINFORCEMENT MESH) ON A 150MM RC BASE SLAB (C30/37 CONCRETE, A252 REINFORCEMENT MESH) — STRUCTURAL ENGINEER TO SPECIFY REINFORCEMENT BASED ON RETAINED HEIGHT AND LATERAL SOIL PRESSURE. FORMS: SHUTTERED IN SITU CONCRETE — OR PERMANENTLY FIXED FORMWORK (MEVA, PERI, OR TWIN WALL — PRE-FABRICATED CONCRETE PANEL SYSTEMS). PRECAST CONCRETE RETAINING UNITS (TOBERMORE OR MARSHALLS PRECAST RETAINING UNITS — LESS COMMON FOR BASEMENT LIGHTWELLS DUE TO DIFFICULTIES OF ACHIEVING WATERTIGHT JOINTS IN A WET LONDON CLAY ENVIRONMENT).

SHORING DURING EXCAVATION: LIGHTWEIGHT HYDRAULIC SHORING (GROUNDFORCE SHORCO OR ACROW FRAME SYSTEM) IS TYPICALLY USED TO SUPPORT THE LONDON CLAY FACE DURING LIGHTWELL EXCAVATION — THE CLAY IS SELF-SUPPORTING FOR EXPOSED FACE DEPTHS UP TO 1.5M WITHOUT SHORING (STIFF LONDON CLAY) BUT SHORING IS REQUIRED FOR DEEPER EXCAVATIONS AND WHERE ADJACENT STRUCTURES ARE AT RISK.

LIGHTWELL WATERPROOFING IS ONE OF THE MOST CRITICAL ELEMENTS OF A SUCCESSFUL BASEMENT CONVERSION IN A LONDON VICTORIAN TERRACE — THE LIGHTWELL IS AN ENCLOSED BELOW-GROUND SPACE OPEN TO THE SKY ABOVE, AND WATER MANAGEMENT (BOTH SURFACE WATER AND GROUNDWATER) MUST BE ADDRESSED COMPREHENSIVELY.

BS 8102:2022 WATERPROOFING STANDARD: ALL BELOW-GROUND STRUCTURES IN THE UK SHOULD BE DESIGNED TO BS 8102:2022 (CODE OF PRACTICE FOR PROTECTION OF BELOW GROUND STRUCTURES AGAINST WATER INGRESS FROM THE GROUND). BS 8102 DEFINES THREE GRADES OF PROTECTION: GRADE 1 (BASIC UTILITY — MOISTURE ACCEPTABLE, E.G. PLANT ROOMS, PARKING), GRADE 2 (BETTER UTILITY — SOME SEEPAGE ACCEPTABLE BUT NO RUNNING WATER — E.G. WORKSHOPS), GRADE 3 (HABITABLE SPACE — NO MOISTURE PENETRATION ACCEPTABLE — E.G. LIVING ROOMS, BEDROOMS). A BASEMENT HABITABLE ROOM IN A LONDON VICTORIAN TERRACE MUST ACHIEVE GRADE 3 PROTECTION.

TYPE A BARRIER WATERPROOFING (TANKING — APPLIED TO THE EXTERNAL OR INTERNAL FACE OF THE STRUCTURE): VANDEX BB75 (CEMENTITIOUS SLURRY CRYSTALLINE WATERPROOFING — APPLIED TO THE INTERNAL FACE OF THE LIGHTWELL WALLS IN TWO COATS — 1.5KG/M²/COAT): ACTIVE CRYSTALLINE SYSTEM — CRYSTALS GROW INTO THE CONCRETE MATRIX IN THE PRESENCE OF WATER, SELF-SEALING SMALL CRACKS. SUITABLE FOR LIGHTWELL INTERNAL FACES WHERE POSITIVE WATER PRESSURE (FROM OUTSIDE THE WALL) IS PRESENT. REMMERS MULTI-TIGHT 2K (2-PART FLEXIBLE CEMENTITIOUS SLURRY — APPLIED TO INTERNAL FACE AT 3-5KG/M²): SIMILAR CRYSTALLINE PERFORMANCE — SUITABLE FOR AREAS OF HIGH MOVEMENT.

TYPE C DRAINAGE AND CAVITY DRAIN SYSTEM: NEWTON SYSTEM 500 (HDPE STUDDED MEMBRANE — NEWTON CDM — FIXED TO INTERNAL FACE OF LIGHTWELL WALLS AND FLOOR WITH STAINLESS STEEL PLUG FIXINGS — CREATES A DRAINAGE CAVITY BETWEEN THE MEMBRANE AND THE SUBSTRATE THAT CHANNELS ANY GROUNDWATER TO A PERIMETER DRAIN): THE MOST COMMONLY SPECIFIED WATERPROOFING SYSTEM FOR LONDON VICTORIAN TERRACE BASEMENT HABITABLE ROOM CONVERSIONS. TYPICALLY COMBINED WITH: NEWTON BASEDRAIN (PERIMETER DRAINAGE CHANNEL — LAID AT THE JUNCTION OF THE FLOOR AND WALL WITHIN THE CAVITY DRAIN MEMBRANE), AND A NEWTON TITAN OR ECLIPSA TWIN PUMP SUMP (AUTOMATIC PUMP — DISCHARGES ANY COLLECTED WATER TO THE DRAINAGE SYSTEM).

EFFECTIVE DRAINAGE OF THE LIGHTWELL FLOOR IS ESSENTIAL — PARTICULARLY IN LONDON WHERE INTENSE SUMMER RAINFALL EVENTS CAN DELIVER 30MM IN AN HOUR (GREATER LONDON AUTHORITY SUDS DESIGN GUIDANCE — THE 1-IN-10-YEAR RETURN PERIOD DESIGN EVENT FOR LONDON).

LIGHTWELL FLOOR DRAINAGE SPECIFICATION: ACO STAINLESS CHANNEL DRAIN (ACO DRAIN STAINLESS X RANGE — EITHER 100MM CHANNEL WITH STAINLESS STEEL LOCKING GRATING OR ACO SELF LINEAR DRAIN — STAINLESS STEEL — FOR A FLUSH CONTEMPORARY LIGHTWELL FLOOR FINISH): CONNECTED TO THE SURFACE WATER DRAINAGE SYSTEM (WHERE A SUITABLE SURFACE WATER SEWER IS AVAILABLE — MOST LONDON VICTORIAN TERRACE FRONT AREAS DRAIN TO A COMBINED SEWER VIA AN EXISTING AREA DRAIN). HEPWORTH SUPERSLEVE CLAY 100MM PIPE CONNECTION FROM LIGHTWELL DRAIN TO EXISTING UNDERGROUND DRAINAGE (WHERE A NEW PIPE RUN IS REQUIRED — TYPICALLY REQUIRED WHERE THE EXISTING DRAIN IS BLOCKED, UNDERSIZED, OR IN THE WRONG POSITION FOR THE RECONSTRUCTED LIGHTWELL). ANTI-FLOODING VALVE: WHERE THE LIGHTWELL DRAIN CONNECTS TO A COMBINED SEWER (WHICH MAY SURCHARGE DURING HEAVY RAINFALL IN LONDON — A KNOWN PROBLEM WITH THE COMBINED VICTORIAN SEWER NETWORK IN INNER LONDON), AN ANTI-FLOODING VALVE (FLODRAIN FULL-BORE ANTI-FLOOD DEVICE OR KESSEL STAUFIX) MUST BE FITTED ON THE DRAIN OUTLET — PREVENTS SEWAGE BACKING UP INTO THE LIGHTWELL AND FLOODING THE BASEMENT.

LIGHTWELL COVERS AND GRILLES: FOR LIGHTWELLS AT FOOTPATH LEVEL (ADJACENT TO THE FRONT BOUNDARY WALL) WHERE PEOPLE COULD ACCIDENTALLY FALL IN: CAST IRON DECORATIVE GRILLE (HERITAGE DESIGN — MATCHING THE ORIGINAL VICTORIAN CAST-IRON AREA GRID — TYPICAL SIZE 600X600MM OR 900X900MM PANELS — ACORN CAST IRON OR ARCHITECTURAL IRONMONGERY FROM CASTCRAFT OR BRINDLEY'S OF BIRMINGHAM): HISTORIC ENGLAND RECOMMENDED RESTORATION APPROACH FOR LONDON CONSERVATION AREA LIGHTWELL GRID COVERS. GALVANISED STEEL BAR GRATING (ALTERNATIVE — RECTAGRATING STANDARD BAR — BS EN 10025 S235 GALVANISED HOT-DIP — SLIP-RESISTANT — LOAD CLASS C250 FOR PEDESTRIAN TRAFFIC).

WHERE A LIGHTWELL CREATES A DROP FROM THE FRONT GARDEN OR FOOTPATH LEVEL TO THE LIGHTWELL FLOOR, SAFETY GUARDING IS REQUIRED UNDER BS 6180:2011 (BARRIERS IN AND ABOUT BUILDINGS) AND APPROVED DOCUMENT K (PROTECTION FROM FALLING, COLLISION, AND IMPACT).

BUILDING REGULATIONS APPROVED DOCUMENT K: A DROP OF MORE THAN 600MM AT THE EDGE OF A LIGHTWELL TO WHICH PEOPLE HAVE ACCESS REQUIRES A BARRIER (GUARD RAIL). WHERE THE EDGE IS AT FOOTPATH LEVEL OR FRONT GARDEN LEVEL ADJACENT TO A PUBLIC AREA: MINIMUM BARRIER HEIGHT: 1100MM (MEASURED FROM FINISHED LEVEL AT THE TOP OF THE LIGHTWELL EDGE). THE BARRIER MUST WITHSTAND THE DESIGN LOADING SPECIFIED IN BS EN 1991-1-1 (EUROCODE 1 — VARIABLE LOADS) FOR BALUSTRADES IN RESIDENTIAL USE (0.74 KN/M HORIZONTAL — LINE LOAD ON HANDRAIL LEVEL). MINIMUM CLIMBABILITY DESIGN: THE RAILING DESIGN MUST NOT PRESENT EASY FOOTHOLDS FOR CLIMBING (HORIZONTAL RAILS AT MID-HEIGHT ARE NOT PERMITTED — VERTICAL MEMBERS OR CONTINUOUS INFILL REQUIRED). FOR A LONDON CONSERVATION AREA PROPERTY: ORIGINAL-STYLE VERTICAL CAST-IRON RAILING WITH SPEARHEAD FINIALS (MATCHING OR COMPLEMENTING THE ORIGINAL VICTORIAN FRONT BOUNDARY RAILINGS) IS THE APPROPRIATE HERITAGE SOLUTION FOR A LIGHTWELL GUARD RAIL. PERIOD IRON RAILINGS: CASTCRAFT LTD (LONDON) OR DIRECT METALWORK LTD — REPRODUCTION VICTORIAN RAILING PANELS IN CAST-IRON OR WROUGHT STEEL FINISHED IN BLACK GLOSS PAINT (OR HOT-DIP GALVANISED AND PAINTED FOR DURABILITY).

GATED ACCESS: WHERE A GATE IS PROVIDED OVER THE LIGHTWELL COVER (OR INTO THE LIGHTWELL FOR DIRECT ACCESS TO THE LOWER GROUND FLOOR), THE GATE SHOULD: BE OF ADEQUATE STRUCTURAL STRENGTH (STEEL TUBE FRAME WITH VERTICAL INFILL — NOT WIRE MESH — BS EN 1991 LOADING). HAVE A LOCKABLE LATCH (STAINLESS STEEL OR GALVANISED STEEL BARREL BOLT + PADLOCK HASP — YALE OR ABLOY PROTEC2) FOR SECURITY. OPEN OUTWARD OR UPWARD (NOT INWARD OVER THE LIGHTWELL OPENING — INWARD OPENING OVER A DROP IS A FALL HAZARD).

BASEMENT LIGHTWELL CONSTRUCTION IN LONDON IS SUBJECT TO BOTH PLANNING RULES AND BUILDING REGULATIONS (PRIMARILY APPROVED DOCUMENT H FOR DRAINAGE, APPROVED DOCUMENT K FOR SAFETY, AND BS 8102 FOR WATERPROOFING — VIA THE BUILDING REGULATIONS COMPLIANCE ROUTE).

PLANNING — NEW LIGHTWELL IN LONDON VICTORIAN TERRACE FRONT GARDEN: A NEW LIGHTWELL (EXCAVATION IN THE FRONT GARDEN OF A LONDON VICTORIAN TERRACE TO CREATE A BELOW-GROUND AREAWAY) IS A MATERIAL CHANGE TO THE EXTERNAL APPEARANCE OF THE PROPERTY AND TYPICALLY REQUIRES PLANNING PERMISSION IF FORWARD OF THE PRINCIPAL ELEVATION. IN MOST LONDON CONSERVATION AREAS: A NEW LIGHTWELL IN A FRONT GARDEN REQUIRES A HOUSEHOLDER PLANNING APPLICATION — AND CONSERVATION OFFICERS WILL ASSESS WHETHER THE LIGHTWELL DESIGN (COVER GRILLE DESIGN, RAILING STYLE, RELATIONSHIP TO THE STREET SCENE) IS APPROPRIATE TO THE CHARACTER OF THE CONSERVATION AREA. THE MOST PLANNING-ACCEPTABLE APPROACH IN A LONDON CONSERVATION AREA: REINSTATE A LIGHTWELL IN THE POSITION AND SIZE OF THE ORIGINAL VICTORIAN AREA (IF ONE EXISTED AND WAS SUBSEQUENTLY FILLED IN), USING HERITAGE MATERIALS (CAST-IRON GRID COVER, PERIOD RAILING).

BUILDING REGULATIONS: A BASEMENT LIGHTWELL AND CONVERSION IS NOTIFIABLE UNDER BUILDING REGULATIONS — THE FULL PLANS ROUTE IS PREFERRED (DRAWINGS SUBMITTED FOR PRE-APPROVAL BEFORE WORK STARTS). KEY APPROVED DOCUMENTS: APPROVED DOCUMENT B (FIRE SAFETY): THE LIGHTWELL WINDOW OF A HABITABLE BASEMENT ROOM MUST ALSO PROVIDE AN EMERGENCY MEANS OF ESCAPE (PART B1 — FIRE ALARMS AND MEANS OF ESCAPE): THE WINDOW MUST BE OPENABLE AND OF ADEQUATE SIZE (MINIMUM 450MM UNOBSTRUCTED HEIGHT, 450MM WIDTH, 0.33M² MINIMUM CLEAR OPENING — APPROVED DOCUMENT B1 RECOMMENDATION) FOR ESCAPE. APPROVED DOCUMENT F (VENTILATION): HABITABLE BASEMENT ROOMS MUST ACHIEVE WHOLE-BUILDING VENTILATION — EITHER THROUGH THE LIGHTWELL WINDOW (IF OPENABLE) PLUS BACKGROUND VENTILATORS, OR VIA A MECHANICAL VENTILATION HEAT RECOVERY (MVHR) SYSTEM.

COSTS: NEW LIGHTWELL IN LONDON VICTORIAN TERRACE FRONT GARDEN (STRUCTURAL RC WALLS, WATERPROOFING, DRAINAGE, RAILINGS — APPROXIMATELY 1.5M X 1.5M X 1.5M DEEP): STRUCTURAL ENGINEER (CALCULATIONS AND DRAWINGS): £800 TO £1,500. EXCAVATION AND RC STRUCTURE (GROUNDWORKS): £3,000 TO £7,000. WATERPROOFING (NEWTON CDM + BASEDRAIN + PUMP SUMP): £1,500 TO £3,000. ACO DRAIN AND CONNECTION: £300 TO £600. CAST-IRON PERIOD RAILING (CASTCRAFT): £600 TO £1,500. TOTAL NEW LIGHTWELL: £6,200 TO £13,600.