New Build Extensions on London Victorian Terraces: Design-Led Planning and Build Process

AN EXTENSION PROJECT ON A LONDON VICTORIAN TERRACE GOES THROUGH SEVERAL DISTINCT DESIGN STAGES:

STAGE 1 — FEASIBILITY AND CONCEPT DESIGN: - SITE APPRAISAL: ASSESS THE SITE CONSTRAINTS — PLOT BOUNDARIES, EXISTING BUILDING LINE, GARDEN DEPTH, NEIGHBOURING PROPERTIES, TREE POSITIONS (AND TREE PRESERVATION ORDERS — CHECK WITH LOCAL AUTHORITY), ACCESS ROUTES, DRAINAGE LOCATIONS - PLANNING CONTEXT CHECK: IS THE PROPERTY IN A CONSERVATION AREA? ANY ARTICLE 4 DIRECTIONS? IS IT LISTED? WHAT HAVE SIMILAR PROPERTIES ON THE STREET ACHIEVED IN PLANNING? - CONCEPT LAYOUT: EXPLORE MASSING OPTIONS, ROOF FORM (FLAT ROOF, PITCHED, MONO-PITCH), GLAZING APPROACH (BIFOLD, SLIDING, PIVOT DOOR — REAR ELEVATION), LANTERN OR ROOFLIGHT POSITION - ARCHITECT: INITIAL HOURLY RATE £80-150/HOUR; FULL DESIGN FEE FOR A STANDARD EXTENSION PROJECT: 8-12% OF BUILD COST (RIBA STAGES 0-7)

STAGE 2 — PLANNING DRAWINGS AND APPLICATION: - ARCHITECTURAL DRAWINGS FOR PLANNING: SITE PLAN (1:500 OR 1:1250), FLOOR PLANS (1:50), ELEVATIONS (1:50 OR 1:100), SECTIONS WHERE REQUIRED, DESIGN AND ACCESS STATEMENT (CONSERVATION AREA APPLICATIONS) - PRE-APPLICATION MEETING: MANY LONDON BOROUGHS OFFER PRE-APPLICATION ADVICE (PAC) — A MEETING WITH THE PLANNING OFFICER BEFORE FORMAL SUBMISSION TO UNDERSTAND LIKELY CONCERNS. COST: TYPICALLY £100-500 (DEPENDING ON BOROUGH). STRONGLY RECOMMENDED FOR CONSERVATION AREA APPLICATIONS OR COMPLEX DESIGNS. - PLANNING APPLICATION: HOUSEHOLDER APPLICATION FEE: £258 (2024 FEES — EXTENSIONS TO EXISTING HOUSES). DECISION PERIOD: 8 WEEKS (STANDARD); LONGER FOR COMPLEX APPLICATIONS OR WHERE THE LOCAL AUTHORITY IS UNDER-RESOURCED.

STAGE 3 — TECHNICAL DESIGN (BUILDING REGULATIONS): - STRUCTURAL ENGINEER DESIGNS: FOUNDATION SPECIFICATION (STRIP FOUNDATIONS, RAFT, OR PADS — DEPENDING ON GROUND CONDITIONS AND STRUCTURAL ENGINEER\'S ASSESSMENT OF THE GROUND); BEAM SIZES FOR STRUCTURAL OPENINGS; FLAT ROOF OR PITCHED ROOF STRUCTURAL DESIGN - SERVICES DESIGN: DRAINAGE (SOAKAWAY OR CONNECTED TO EXISTING DRAIN — BUILDING CONTROL REQUIRES CONNECTION TO EXISTING DRAIN IF A SEWER IS WITHIN 100M); ELECTRICS DESIGN; HEATING EXTENSION IF RELEVANT - BUILDING REGULATIONS DRAWINGS: DETAILED CONSTRUCTION DRAWINGS SHOWING WALL BUILD-UP (WITH U-VALUES), FOUNDATION SECTION, WINDOW AND DOOR SCHEDULES (WITH U-VALUES), ROOF SECTION (WARM FLAT ROOF BUILD-UP OR PITCHED ROOF), VENTILATION AND DRAINAGE

STAGE 4 — SPECIFICATION AND CONTRACTOR PROCUREMENT: - SPECIFICATION DOCUMENT: SETTING OUT THE MATERIALS AND WORKMANSHIP STANDARDS FOR EACH TRADE — BRICKWORK (BRICK MATCHING SPECIFICATION), ROOF WATERPROOFING PRODUCT, GLAZING SPECIFICATION (U-VALUE, COLOUR, HARDWARE), INTERNAL FIT-OUT SPEC - CONTRACTOR SELECTION: INVITE 2-3 CONTRACTORS TO PRICE AGAINST THE ARCHITECT\'S DRAWINGS AND SPECIFICATION. BEWARE OF UNSPECIFIED WORK — QUOTES WITHOUT A PROPER SPECIFICATION ARE NOT COMPARABLE AND MAY OMIT SIGNIFICANT ITEMS. - CONTRACT FORM: A JCTMB (JOINT CONTRACTS TRIBUNAL MINOR WORKS BUILDING CONTRACT) OR HOMEOWNER CONTRACT IS RECOMMENDED — SETS OUT PROGRAMME, PAYMENT TERMS, VARIATION PROCEDURE, AND DEFECTS LIABILITY

STRUCTURAL SYSTEMS FOR LONDON VICTORIAN TERRACE EXTENSIONS:

FOUNDATIONS: - STRIP FOUNDATIONS (THE DEFAULT FOR MOST SINGLE-STOREY REAR EXTENSIONS IN LONDON CLAY): TRENCH-FILL CONCRETE (C25 MIX) POURED INTO MACHINE-DUG TRENCHES. DEPTH DEPENDS ON TREE ROOT PROXIMITY (NHBC STANDARDS TABLE — POPLAR WITHIN 10M REQUIRES FOUNDATIONS TO 2.5-3.0M IN LONDON CLAY), FROST DEPTH (MINIMUM 600MM), AND STRUCTURAL ENGINEER SPECIFICATION. - PILE AND BEAM (WHERE TREES ARE CLOSE OR GROUND CONDITIONS ARE VARIABLE): BORED MINI-PILES WITH REINFORCED GROUND BEAMS — MORE EXPENSIVE THAN STRIP FOUNDATIONS BUT APPROPRIATE WHERE SOIL IS DISTURBED OR TREE ROOTS ARE A RISK.

WALLS: - CAVITY WALL (THE MOST COMMON FOR RESIDENTIAL EXTENSIONS): 102MM EXTERNAL BRICK SKIN + CAVITY (100MM TOTAL — TYPICALLY 50MM CLEAR CAVITY + 50MM PARTIAL FILL CAVITY INSULATION OR FULL-FILL WITH EARTHWOOL CAVITY BATT) + 100MM LIGHTWEIGHT BLOCKWORK (AGGREGATE OR THERMAL BLOCK) + 12.5MM PLASTERBOARD ON DOT AND DAB. TOTAL WALL U-VALUE TARGET: ≤ 0.18 W/M²K. - SIP PANELS (STRUCTURAL INSULATED PANELS): FASTER THAN TRADITIONAL MASONRY; GOOD THERMAL PERFORMANCE; MORE COMMON IN CONTEMPORARY ARCHITECTURAL EXTENSIONS. STRUCTURALLY ENGINEERED; ARRIVE ON SITE PRE-CUT — FRAME GOES UP IN 1-2 DAYS. - STEEL FRAME (PORTAL FRAME OR COLUMN/BEAM GRID): WHERE LARGE SPANS, TALL GLAZING, OR CANTILEVER ELEMENTS ARE REQUIRED — PARTICULARLY IN CONTEMPORARY ARCHITECTURAL EXTENSIONS WITH LARGE GLASS WALLS.

SPECIFICATION LEVELS AND INDICATIVE COSTS (SINGLE-STOREY REAR EXTENSION, 20M², LONDON 2025-2026): STANDARD BUILD (BRICK + BLOCK CAVITY WALL, EPDM FLAT ROOF, UPVC WINDOW/DOOR — ADEQUATE BUT NOT PREMIUM): £55,000-75,000 QUALITY BUILD (BRICK + BLOCK CAVITY, SARNAFIL FLAT ROOF, ALUMINIUM THERMALLY-BROKEN BIFOLD DOORS, TILED FLOOR, QUALITY PLASTER + DECORATION): £75,000-100,000 PREMIUM BUILD (BRICK + BLOCK OR SIP FRAME, ZINC OR TECTIVA CLADDING FLAT ROOF, SCHUCO SLIDING DOOR, FEATURE CEILING, ENGINEERED OAK FLOOR): £100,000-150,000+ DOUBLE-STOREY REAR EXTENSION (20M² EACH FLOOR — 40M² TOTAL): ADD 40-60% TO SINGLE-STOREY COST FOR THE UPPER FLOOR AND STAIRCASE WRAP-AROUND (L-SHAPED, 25M²): £80,000-130,000 (SEE WRAP-AROUND GUIDE FOR DETAIL)