How to Write a Building Project Specification

These two documents are closely related but serve different purposes:

**Scope of works**: Describes WHAT will be done — the extent and list of work packages. Example: 'Rear extension: demolition of existing lean-to, new foundations, cavity wall construction, flat roof, bi-fold doors.'

**Specification**: Describes HOW each element will be done and to what standard — the materials, methods, and quality expectations. Example: 'Cavity walls to be constructed in facing brick (to match existing — see brick specification note) outer leaf, 100mm Kingspan Kooltherm K5 full-fill cavity insulation, 100mm dense aggregate blockwork inner leaf. Cavity wall ties to be stainless steel Ancon S1 at maximum 900mm horizontally and 450mm vertically. Mortar to be 1:1:6 cement:lime:sand mix in Natural Grey finish.'

The scope tells the contractor what they are doing; the specification tells them how. Both are needed for a properly specified and consistently priced project.

• **Common problems when there is no specification**:
• •Contractor A prices 75mm polystyrene bead cavity fill; Contractor B prices 100mm PIR full-fill; Contractor C prices no cavity insulation. The three quotes are incomparable even though they all say 'cavity wall extension'
• •A dispute arises at completion about the mortar colour: the client wanted Buff mortar to match the existing house; the contractor used Grey because nothing else was specified
• •An argument about the number of coats of paint in the specification — 'decorated throughout' is interpreted by the contractor as one mist coat on new plaster; the client expected two full coats of emulsion
• •The contractor omits the structural engineer's specified galvanised padstones under the steel beam, substituting a softer block; the Building Control inspector flags this at the inspection stage, requiring remedial work

A building specification for a domestic extension or refurbishment typically includes the following sections:

• **1. General conditions**:
• •Site management standards (tidiness, waste management, working hours)
• •How variations will be managed (written confirmation required before any variation is instructed)
• •Programme expectations and notification of delay
• •Insurance requirements (PLI certificate, Employers' Liability)
• •Building Regulations approval basis

• **2. Demolition and enabling works**:
• •What is to be demolished
• •How demolition waste is to be managed (skip, licensed carrier, salvage requirements)
• •Any structural investigation required before demolition commences
• •Asbestos survey requirement for pre-2000 structures

• **3. Groundworks and drainage**:
• •Foundation type, dimensions, and concrete specification (e.g., C25/30 ready-mixed, 150mm cover to reinforcement)
• •DPC specification and position
• •Drainage: pipe material (clay, UPVC), diameter, gradient, inspection chamber specification, connection to existing drainage
• •Formation levels, sub-base specification (MOT Type 1 150mm compacted), blinding (50mm sharp sand)

• **4. Superstructure (walls)**:
• •External wall construction: brick/block/frame specification, cavity width, insulation type and thickness, cavity tie specification
• •External brick: type (facing brick, common brick), colour, bond pattern, mortar mix and colour
• •Internal blockwork: density, thickness

• **5. Roof structure and covering**:
• •Roof type (flat, pitched, mono-pitch)
• •Flat roof: falls specification (1:60 to outlets), membrane type and system (GRP, TPO, cold-applied bitumen, Bauder warm roof), insulation type and thickness (PIR 120mm minimum for flat roof), ventilation requirements
• •Pitched roof: tile or slate specification, underlay type, batten, ridge and hip details
• •Rooflights: manufacturer (Velux, Fakro), glazing specification (double or triple, low-E, obscure where required)

• **6. First-fix MEP (Mechanical, Electrical, Plumbing)**:
• •Electrical: circuits to be provided (number, types), consumer unit capacity requirement, cable specification (6242Y twin-and-earth where exposed), trunking where required
• •Plumbing: pipe material (copper, PEX), hot and cold supply routes, waste pipe specification and gradients
• •Heating: radiator sizing (or UFH specification), connection to existing boiler or new boiler specification, controls

• **7. Insulation and airtightness**:
• •Insulation specification for walls, floor, and roof (U-value targets and product specification)
• •Vapour control layer position and product
• •Airtightness: tape and membrane system at all junctions, target air permeability for SAP assessment

• **8. Internal plastering and drylining**:
• •Plaster system (sand/cement scratch coat, Hardwall, Carlite finish skim, or dot-and-dab drylining)
• •Skim finish (two-coat or single coat, sponge-floated or steel-trowel finish)
• •Fire-rated plasterboard where required (type, thickness, number of layers)

• **9. Carpentry and joinery**:
• •Door specification: type (solid core, hollow core), size, finish (primed for painting, pre-finished)
• •Fire door specification: FD30S, self-closer, certificate number
• •Skirting and architrave: profile, material, dimensions
• •Staircase: stringer type, newel, balustrade specification (if applicable)

• **10. Tiling**:
• •Adhesive and grout specification (flexible vs. standard; waterproof vs. standard)
• •Tile pattern (brick bond, grid bond, herringbone)
• •Grout joint width
• •Note: tiles are client-supplied; RCB installs client-supplied tiles to the specification agreed

• **11. Finishes, painting, and decoration**:
• •Number of coats of paint in each location
• •Paint products (where known or specified)
• •Treatment of new plaster (mist coat requirement, drying time before finish coat)

• **12. Exclusions and client-supplied items**:
• •All items not included in the contractor's scope
• •Items supplied by the client and installed by the contractor (kitchen units, tiles, sanitaryware, light fittings)
• •Items supplied and installed by others

**The specification and the quote**: When a full specification is provided alongside the scope of works, the quotation is a response to an exact brief. When quotes are received against the same specification, they are genuinely comparable — any difference in price is in the contractor's margin, labour rate, or preliminary costs, not in the specification of materials.

• **Who prepares the specification**:
• •Architect or designer (for professionally designed projects — the specification is part of the tender package alongside drawings)
• •Quantity surveyor (for larger projects where a full bills of quantities is produced)
• •The contractor themselves (many contractors prepare their own specification as part of the quote — the client should review this carefully to ensure it matches their expectations)
• •A design-and-build contractor's project coordinator (the specification is developed alongside the client during the pre-construction stage)

**How detailed the specification needs to be**: The level of detail required is proportional to the project value and complexity:

*Simple refurbishment (under £25,000)*: A clear scope of works with key material decisions noted (tile adhesive specification, paint coats, floor finish) is usually sufficient. The contractor will fill in the gaps — the client should review the quote to confirm the assumptions match expectations.

*Medium-complexity extension (£50,000–£150,000)*: A formal specification document covering all the sections above is appropriate. The architect or project manager should produce this as part of the design package before tender.

*Complex or high-value project (over £150,000)*: A full specification, schedules of materials and fittings, and potentially a bill of quantities are appropriate. The client should engage a cost consultant or quantity surveyor to prepare the tender documents.

• **The client's minimum responsibilities**:
• Even where the contractor prepares most of the specification:
• •Confirm the external finish and facing materials in writing before construction starts
• •Confirm mortar colour before masonry begins (changing mortar colour after blockwork has been laid requires demolition and rebuilding)
• •Confirm tile pattern and grout colour before any tiling begins
• •Confirm paint finish and colour scheme before decoration begins

These four decisions, if not made explicitly before the relevant work stage, routinely become the source of specification disputes at or near project completion.