Writing a Specification for Your London Extension: What to Include and Why It Matters

**What happens when there is no specification**:

The most common source of disputes in London residential construction projects is not price disagreements or contractor competence — it is ambiguity about what was included in the original agreement. Without a specification:

• •Two contractors pricing the same project will make entirely different assumptions about the quality of materials, the finishes to be used, and the extent of work — their quotes are not comparable on a like-for-like basis
• •The contractor will fill in the gaps in the brief with 'standard practice' — which may or may not match what the homeowner expected or assumed was included
• •During the works, the homeowner says 'I thought that was included' and the contractor says 'that was never agreed' — leading to costly variations and deteriorating relationships
• •At the end of the project, disputes about the quality of finishes or the completeness of the works are very difficult to resolve without a written standard to refer to

**What a specification does**:

• A specification creates a clear, shared understanding between homeowner and contractor about what is being built, to what standard, and with what materials. It:
• •Enables like-for-like comparison of contractor quotes (all contractors price the same spec, not their own assumed standard)
• •Defines the quality standard — if the spec says 'porcelain tiles to kitchen floor — client to supply', the contractor cannot substitute ceramic tiles; if it says 'full cavity fill PIR insulation to walls', the contractor cannot substitute mineral wool
• •Protects both parties — the homeowner has a benchmark for the quality they are entitled to; the contractor has a clear scope that protects them from unreasonable expectations
• •Reduces variation cost claims — if it is in the spec, it is in the contract price; if it is not in the spec, the contractor can reasonably claim it as a variation

**A specification is different from drawings**:

Architectural drawings show WHERE things are — the layout, dimensions, and appearance of the building. The specification says WHAT things are made of — the materials, products, standards, and workmanship quality for each element.

Both are needed for a complete building contract. A drawing without a specification says 'build a flat roof here' — the spec says 'warm flat roof: 150mm Kingspan TR26 PIR, 18mm OSB3 deck, Bauder torch-on membrane system with 20-year guarantee, falls to achieve minimum 1:80 to outlets'.

**The two types of specification**:

*1. Performance specification*: Describes the performance required of an element without specifying exactly how it is achieved. Example: 'External wall insulation to achieve minimum U-value of 0.26 W/m²K — contractor to propose and confirm specification prior to construction. Building Control approval of specification required before work commences.'

*2. Prescriptive specification*: Specifies exactly what products and materials are to be used. Example: 'External wall cavity insulation: Knauf Eco-Roll 100mm (full fill) to full height of cavity — insulation certificate to be provided to Building Control.'

For residential extensions, a combination of both is common — performance specifications for elements where the contractor has design responsibility, and prescriptive specifications for finishes, fittings, and products that the homeowner has chosen or where quality is important.

**The specification checklist for a typical London rear extension**:

A typical specification for a London rear kitchen extension covers the following sections — each of which should be described in sufficient detail to enable contractors to price consistently:

• **1. Demolition and preparation**:
• •What is to be demolished (existing rear wall masonry — full or partial? existing lean-to? existing rear path or paving?)
• •How demolished material is to be disposed of (skip hire, contractor to arrange; waste transfer note required)
• •Whether any existing services (gas, electric, water, drainage) need to be diverted and by whom
• •Assumed condition of the existing building — any unusual conditions (asbestos, contaminated materials) discovered after start of works are a potential variation

• **2. Foundations**:
• •Foundation type (strip footing, piled, beam and block void)
• •Concrete specification (C25/30 or C30/37 reinforced as per SE's design)
• •Foundation depth (from SE's drawings)
• •DPC and DPM provision (150mm above external ground level minimum; DPM lapped to underfloor membrane)
• •Note: 'Foundation type and design to structural engineer's specification — SE to be appointed by [client/contractor]'

• **3. External walls**:
• •Cavity wall specification: outer leaf (brick type — London stock, red brick, or other), inner leaf (100mm concrete block), cavity width, insulation type and thickness, ties, cavity closers at reveals and heads
• •Example: 'Cavity wall: outer leaf 102.5mm facing brick (Ibstock Tradesman London stock or approved equal — sample panel to be approved by client before construction); 100mm cavity with 100mm Knauf Eco-Roll full-fill mineral wool cavity slab; 100mm medium density concrete block inner leaf; 3mm flush jointed mortar'
• •Lintels over openings (specify product — Keystone Hi-Therm or equivalent insulated cavity lintel; size to SE's calculation)
• •External render or cladding where applicable

• **4. Roof**:
• •Flat roof: warm roof build-up, PIR thickness (minimum 150mm as per Part L — see `building-regulations-part-l-guide`), deck type (OSB3), waterproofing system and guarantee (EPDM single-ply or torch-on felt — specify manufacturer and required guarantee period), drainage outlets (number, type, position), upstand height (minimum 150mm to walls)
• •Pitched roof: rafter size and span, insulation strategy (between rafter, over rafter), roofing material, tile type (specify — clay or concrete, manufacturer where known), underlay (Roofshield or BS5534-compliant breathable membrane), batten gauge, ridge tile type
• •Rooflights: manufacturer, model, size, glazing specification (triple glazed — Uw ≤ 1.1 W/m²K)

• **5. Ground floor**:
• •Floor build-up: Type 1 sub-base (150mm), DPM (1200 gauge polythene), insulation (100mm PIR — minimum for Part L 0.25 W/m²K compliance — see `concrete-floor-guide`), reinforced concrete slab (125mm C20 with A142 mesh), screed (65mm sand-cement or 50mm anhydrite liquid screed if UFH)
• •UFH if included: barrier polyethylene pipe, manifold, pressure test before screed
• •Note: 'Floor finish to be client-supplied' or specify floor finish if contractor-supplied

• **6. Internal walls and partitions**:
• •Specify any new internal walls (stud partition with acoustic mineral wool between, or block)
• •Fire-rated construction where required (party wall, HMO, garage-to-house interface)

• **7. Glazing and external doors**:
• •Bi-fold or sliding doors: frame material (aluminium — specify colour RAL, powder coat or anodised finish), glazing specification (double or triple, glass specification, solar control if south-facing), hardware (lock type, threshold type — level to Part M requirements), manufacturer or performance specification (Uw ≤ 1.4 W/m²K whole unit)
• •Trickle ventilators: specify compliance with Part F — trickle ventilators in all new windows (4,000mm² EA per habitable room)

• **8. Electrical first fix**:
• •Number and location of socket outlets (client to mark up drawing)
• •Lighting circuits (downlights — specify IP rating if in bathroom/shower zone)
• •Extract fan provision (kitchen — 60 l/s, bathroom — 15 l/s; Part F)
• •Consumer unit upgrade if needed
• •EV charging provision if applicable (Part S)
• •All new electrical work to be by registered electrician (NICEIC or NAPIT); electrical installation certificate to be provided

• **9. Plumbing first fix**:
• •New boiler if needed, or connection to existing
• •UFH system if included (see floor section)
• •Kitchen sink supply and waste
• •Downpipe and surface water drainage (see `drainage-extensions-guide`)
• •All gas work by Gas Safe registered engineer; gas safe certificate to be provided

• **10. Plastering and internal finishes**:
• •Plaster specification: two-coat hardwall plaster or lightweight plaster to all new masonry walls; skim coat to plasterboard ceilings and stud partitions
• •Tolerance standard: plaster flat within 3mm in 1.5m (straight-edge test)
• •Coving or shadow gap detail where applicable

• **11. Second-fix and finishes (note standard RCB position)**:
• •'Second-fix materials (kitchen, bathroom fittings, floor finishes, internal door hardware, light fittings, sockets/switches) to be supplied by client unless specifically included in this specification'
• •If contractor-supplied: specify category (e.g., 'mid-range kitchen from [manufacturer] — full schedule of units and appliances to be provided by client to contractor for costing')

• **12. Decoration**:
• •Prime coat and two coats emulsion to all new plasterwork (if included — some homeowners redecorate themselves)
• •Specify if gloss or eggshell to woodwork, and colour basis (neutral/white as base or specific colour)

• **13. Exclusions and assumptions**:
• •'Client to supply all second-fix materials unless stated above'
• •'No carpet or vinyl floor coverings — RCB does not install carpet'
• •'Party wall notice and surveyor costs (if applicable) to be handled by client'
• •'Scaffold (if required) for neighbour access — to be agreed'
• •'Asbestos removal (if encountered) — additional cost item'
• •'Any additional drainage connections beyond [scope described above] are a variation'

**Getting comparable quotes from multiple contractors using a specification**:

The primary commercial benefit of a specification is the ability to compare quotes on a like-for-like basis. Without a specification, a quote from Contractor A (who assumes mineral wool insulation, standard double glazing, and a basic felt flat roof) cannot be compared to a quote from Contractor B (who assumes PIR insulation, triple glazing, and a 20-year guaranteed EPDM membrane) — the prices will be different but the scope is different too.

• With a shared specification:
• •All contractors price exactly the same scope
• •Price differences reflect different labour rates, margin, or procurement costs — not different scope assumptions
• •The lowest price is genuinely the lowest price for the same outcome

• *How to use the specification when requesting quotes*:
• •Send the specification and drawings to all contractors at the same time
• •Ask each contractor to confirm that their price is based on the specification as written
• •Ask each contractor to explicitly list any items they have excluded or where they have priced a substitution (and the reason)
• •When comparing quotes, cross-check the inclusions and exclusions against the specification line by line

**Variations during the project — how the specification controls them**:

• Once the contract is signed, any change to the agreed specification is a variation. Variations should be:
• •Agreed in writing before the additional work is started
• •Priced by the contractor as a variation cost (additional cost or saving compared to the original scope)
• •Incorporated into an updated contract value if significant

• The specification is the reference point for determining what is and is not a variation:
• •If the spec says 'Kingspan TR26 150mm PIR flat roof insulation' and the contractor proposes substituting 100mm, that is a specification change — not a variation (it is a reduction from what was agreed). The contractor should not be able to claim this change saves them money unless the homeowner agreed to amend the specification.
• •If during construction it is discovered that the existing drainage run is in a different location than assumed, requiring additional drainage work not covered in the spec — this IS a legitimate variation.

**RCB Design & Build's standard specification positions**:

RCB Design & Build uses the following standard specification positions in all proposals and contracts (unless specifically varied for a particular project):

• •*Second-fix materials*: All second-fix materials (kitchen units, bathroom sanitaryware, floor finishes, internal door hardware, sockets, switches, light fittings) are client-supplied unless specifically included in the written specification for the project
• •*Carpet*: RCB does not install carpet. Carpet supply and installation is excluded from all RCB contracts
• •*Detailed estimate*: A detailed estimate for a project is only produced after a site visit and receipt of architectural drawings and any other relevant information (structural engineer's drawings, drainage surveys) — a ballpark figure provided before this stage is indicative only and should not be relied upon as a contract price
• •*Exclusions assumed unless specified*: Furniture removal and storage; external works beyond the immediate extension footprint (unless specified); specialist cleaning after completion; building insurance upgrade (client to notify insurer)

**Common items London homeowners forget to specify (and that cause disputes)**:

1. *Bi-fold/sliding door threshold detail*: The threshold type (flush to floor or stepped), whether a drop-sill, rebated threshold, or channel drain is used, and the waterproofing detail at the junction of the extension floor and the threshold — all need to be specified. A flush level threshold (Part M requirement) requires careful coordination between the floor level inside and the patio level outside.

2. *External patio or ground levels*: Whether the contractor is responsible for laying the external patio at the correct level to coordinate with the level threshold, or whether the patio is the client's separate responsibility after the extension is complete.

3.*Extract ductwork route and termination*: The kitchen extract fan duct must terminate at the external wall or roof — the route and the external grille type should be specified. Ducts that terminate into a roof space (common and incorrect — see `building-regulations-part-f-guide`) are a Building Control failure.

4. *Fire alarm system*: The type of smoke alarm system (optical, heat, combined), the interconnection method (wired or wireless), the zones required by Building Control, and where alarms are to be located.

5. *Making good after structural works*: When a structural beam is installed in an existing opening (e.g., the opening from the existing kitchen into the new extension), the existing wall needs to be made good above and around the beam. This includes replastering, painting, and dealing with any pipe or cable routes that were buried in the original wall. Who does this, and to what standard, is often not specified.