MEP Bills of Quantities: Why Most Are Wrong Before Work Starts

MEP cost overruns can be disproportionate to the apparent size of the original design change. The Bill of Quantities is often where the problem begins: quantities are measured from an immature design, descriptions do not match the specification and responsibilities are left between packages. A competitively priced tender can then conceal very different assumptions about what each bidder has included.

The bill should make scope comparable and commercially manageable. It cannot complete an unfinished design, but it can expose uncertainty before that uncertainty becomes a claim. Reviewing it before tender is usually cheaper than reconstructing what everyone intended once installation has started.

Measurement methodology — NRM2 vs SMM7 vs bespoke

Agree the measurement rules before measuring. NRM2 provides detailed measurement rules for building works, including building services; older documents may still refer to SMM7. CESMM4 is principally a civil engineering method and should not be treated as an interchangeable default for building MEP packages. NRM2 Part 3 is the tabulated rules for building works, not a standalone mechanical-only standard. State the edition, any departures and the boundary between measured items and deemed inclusions.

For pipework, identify material, diameter, insulation requirements, fittings and relevant installation conditions. Ductwork needs consistent descriptions of size, material, pressure class and treatment of fittings and insulation. Equipment descriptions should tie to schedules and define associated connections and controls. Containment requires a clear basis for sizes, routes, supports and accessories. The objective is not maximum detail for its own sake: it is ensuring bidders measure and price the same obligation.

Bespoke schedules often combine an entire system into one line and rely on a broad 'complete installation' description. That can be appropriate for a clearly defined performance package, but it is a poor substitute for missing information. A specification describes required performance; it does not necessarily resolve quantities, installation conditions or trade interfaces.

The scope gap problem

Many MEP bills are prepared from outline rather than coordinated detailed design. Common omissions include builders work, cutting and patching, fire stopping interfaces, testing and balancing, commissioning attendance, O&M documentation and as-built surveys. Each may appear minor beside a major plant order, yet collectively they can represent a substantial allowance. A working risk allowance of 8–15% is sometimes used for such unresolved scope, but it is not a universal benchmark: quantify the exposure for the actual project rather than presenting a range as measured fact.

Create an explicit responsibility matrix. Who forms an opening, who sleeves it, who seals it and who verifies the fire strategy? Who supplies temporary power for commissioning? Is witnessing by the designer included? A bill that names each interface gives the project team something practical to procure and administer.

Provisional sums used as a substitute for design

Provisional sums should be used sparingly where scope genuinely cannot yet be defined, with their status and adjustment mechanism stated. An allowance for BMS, sprinklers or life safety systems is not evidence that the package is affordable. If the design is incomplete, a low provisional figure merely postpones the budget decision.

Risk allocation depends on the contract and whether the sum is defined or undefined; it does not automatically transfer all design or price risk to the contractor. Set out the assumptions, design deliverables, instruction date and programme allowance. Obtain specialist budget quotations where possible, and reconcile each sum before award rather than leaving an orphaned allowance with no owner.

Equipment supply vs install — the split that catches clients out

Client-supplied plant can offer purchasing advantages, but splitting supply from installation creates an interface that must be priced and managed. Allocate unloading, storage, lifting, protection, final connections, controls integration, commissioning and manufacturer attendance. Identify who checks compatibility before the order is placed.

Warranty conditions may depend on approved installation or a manufacturer's start-up visit. Responsibility for transit damage is different from damage after delivery or during installation. If these obligations are not explicit, the eventual resolution may require a variation or Compensation Event where the contractual conditions are met. A clear interface schedule is much more useful than arguing later about whether 'install' meant 'make operational'.

What a robust MEP BoQ looks like

• An agreed measurement methodology, with departures and design information clearly recorded.
• Separate mechanical, electrical and public health scopes, with cross-package interfaces reconciled.
• Provisional sums with an owner, a stated basis and a plan for replacement by defined scope.
• Explicit equipment supply, installation, controls and warranty responsibilities.
• Testing, commissioning, witnessing, training and handover deliverables included.
• Agreed daywork rates and percentage additions where the contract uses them, without confusing these with the contract's normal change valuation rules.