How to Write a Specification for a London Home Extension

A specification is a written document that describes the standard and quality of work to be carried out on the project. It supplements the drawings (which show what to build and where) by describing how to build it and to what quality standard. The specification answers questions the drawings cannot: What type of brickwork? English bond or stretcher bond? What mortar colour and joint profile? What insulation type and thickness? What plasterboard board thickness? What paint system — how many coats? What is the standard of finish in the bathroom? Are the kitchen units included or client-supplied? What specification of windows — double or triple glazed? What type of flooring, and who supplies it? Without a written specification, all these questions are decided during construction — and the answers are decided by the contractor on the basis of what they priced (which may be the cheapest option), not what the homeowner expected. Common specification disputes on London extensions: Client expected: porcelain tiles to the full bathroom wall height. Contractor quoted: tiling to 1.8m splashback height. Value difference: £1,500. Client expected: oak internal doors throughout. Contractor quoted: standard hollow-core doors. Value difference: £2,000. Client expected: underfloor heating to the kitchen extension. Contractor quoted: radiators (UFH not explicitly specified). Value difference: £3,500. A specification eliminates these disputes because the standard and inclusion are defined before the contract price is agreed. The contractor prices what is specified — and if they propose a substitution or value engineer a specification item, they must do so explicitly and the homeowner can accept or reject the substitution with knowledge of what they are agreeing to.

A practical specification for a London home extension or renovation is structured to mirror the construction sequence — from groundworks and structure through to finishes and completion. Key sections: Demolition and enabling works: what is to be demolished and removed; whether asbestos surveys are required (required before demolition of any pre-2000 structure in London); specification for structural propping and temporary support during demolitions. Groundworks and foundations: type of foundation (strip, pad, raft, mini piles) as specified by the structural engineer; concrete specification (strength class, mix, minimum cover); drainage: type of drainage (clay, plastic, or concrete), gradient, and connection points. Structural works: structural steel specification (grade, surface treatment, fire protection); structural masonry (block type, mortar designation, tie specification); structural frame (if timber frame — engineered timber specification, preservative treatment, structural connections). External envelope: external walls — brick type, mortar colour, joint profile (bucket handle, flush, recessed); internal face of external wall — block type, cavity width, insulation specification and thickness, cavity closers and ties; flat roof build-up — structural deck specification, VCL, insulation type and thickness, waterproofing system (EPDM, GRP, or other) and manufacturer; pitched roof — rafter and purlin sizes, breathable membrane, counter batten and tile batten, tile or slate specification and colour, ridge and hip details; windows and external doors — frame material (PVCU, aluminium, timber), glass specification (double or triple, low-E coating, argon fill), hardware specification, colour/finish. First-fix services: plumbing — pipework type (copper, plastic, push-fit), hot water system (combi boiler extension, unvented cylinder, or other), radiator type and finish, underfloor heating (if specified); electrics — wiring and cable specification, consumer unit type, socket and switch type and finish (standard white or designer). Insulation and airtightness: roof insulation to achieve target U-value; wall insulation type and target U-value; floor insulation (over ground floor slab) type and target U-value; airtightness specification (target air permeability in m³/h/m²) if applicable. Plastering and internal walls: plaster type (bonding + finish, multifinish, board finish), number of coats, tolerance (in mm at 2m straight edge); plasterboard type and thickness (12.5mm standard, 15mm acoustic or fire, 19mm on metal studwork); partition walls — timber stud or metal stud, specification of noggins, acoustic insulation in partitions between rooms. Second fix and finishes: internal doors — door type (solid oak, hollow core, fire door), door furniture (hardware specification — material, finish, lever or knob); flooring — type, manufacturer/range if specified, underlay, whether supply is by client or contractor; skirting and architrave — profile and material; decoration — paint system and number of coats, paint type (emulsion, eggshell, gloss), colour specification if agreed; bathroom — wall tile specification (size, manufacturer, grout colour), sanitary ware specification (manufacturer and model), heated towel rail specification; kitchen — kitchen supply and install by client or contractor, worktop specification, upstand, splashback.

The exclusions and assumptions section of the specification is the most commercially important part. It makes explicit what is NOT included in the contract scope — and what the contractor has ASSUMED in pricing the work. This section protects both parties: it protects the contractor from claims that they should have included items that were not specified; and it protects the homeowner by making the contractor's assumptions explicit and challengeable before the contract is signed (not during construction when the leverage is gone). Typical exclusions and assumptions for a London extension specification: Supply items: are the kitchen units, bathroom sanitary ware, floor tiles, and internal doors supplied by the homeowner or by the contractor? (Make explicit.) Second-fix materials: RCB Design & Build standard contract terms specify that all second-fix materials (floor tiles, wall tiles, sanitary ware, kitchen units, internal doors, ironmongery, light fittings) are client-supplied unless explicitly stated otherwise in the specification. Make good to existing: does the scope include making good to existing internal finishes (redecoration of adjacent rooms affected by the works, making good to plastering and flooring that is disturbed by first-fix services routing)? Or is the scope limited to the new extension and the immediate opening through the existing house wall? External works: is external landscaping included (levelling and replanting the garden disturbed by the groundworks, resurfacing of paths and patio areas affected by the extension)? Or are external works excluded? Architect and structural engineer fees: are professional fees (architect, structural engineer, party wall surveyor) included in the contractor's quotation, or are they separate client-side costs? Building Control application fee: is the Building Control application fee (typically £500-£1,500) included in the contractor's quotation, or is it a client-side cost? Provisional sums: items whose cost cannot be determined until further information is available (for example, the cost of party wall works, or the cost of connecting to the main drain if the drain depth is not yet confirmed) should be identified as provisional sums with an estimate — not included in the fixed price. Party wall works: are party wall surveyor fees, the Schedule of Condition, and any remedial works to the Adjoining Owner's property included in the contract scope?

The specification is most valuable as the basis for competitive tendering — the process of obtaining quotes from two or three contractors for the same scope of work. Without a specification, contractors produce quotes based on their own assumptions about the specification. The result is quotes that are not comparable — one contractor has priced high-quality brickwork, the other has priced the cheapest available; one has included UFH, the other has not. Comparing these quotes and choosing the cheapest is likely to result in either a poor quality outcome or a significant variation claim during construction. With a specification, all contractors price the same scope and the same standards — the quotes are genuinely comparable. Process for specification-based tendering in London: Prepare drawings (plan, elevation, section at minimum) and the specification document (using the structure above). Issue both to all contractors simultaneously. Ask each contractor to price the specification and to identify any items they wish to price as an alternative or deviation from the specification. At tender return, compare the quotes line-by-line (not just the total) — identify where deviations or omissions explain price differences. Select the contractor on the basis of the full picture: price, programme, qualifications, references, contract basis, and your confidence in their ability to deliver. A clear specification with an exclusions and assumptions section should produce quotes that are accurate to within 10-15% of the final contract value — significantly better than the 30-50% range of inaccuracy that is common with specification-free quote processes in London.