How to write an MEP specification in the UK
Who writes the building services specification, how CAWS groups R to Y compare with Uniclass Ss and Pr, when to specify by performance or by product, which standards are moving in 2026, and where coordination with the architectural specification breaks down.
How do you write an MEP specification in the UK?
You set out the mechanical, electrical and public health systems the project needs, what each must achieve, and which parts the designer fixes versus what the contractor designs, organised by CAWS work sections (mainly groups R to Y) or Uniclass 2015 systems (Ss) and products (Pr). It then has to be coordinated line by line with the architectural specification and drawings, because most services problems on site sit at the interfaces.
On most projects of any size, the building services engineer (the M&E consultant) writes the MEP specification. The architect writes the architectural specification and leads coordination between the two. How much design the consultant does, and how much passes to the specialist contractor, depends on the procurement route.
BSRIA's Design Framework for Building Services (BG 6/2018, 5th edition) is the usual reference for agreeing who designs what, with activity pro-formas and drawing and model definitions.
Since 1 October 2023, everyone doing design work in England also has duties under Part 2A of the Building Regulations 2010, inserted by SI 2023/911. Regulation 11J sets the general duty to plan, manage and monitor design work so that it complies, and regulation 11K adds duties for designers. A services specification is part of that design work, so a vague or contradictory one is a compliance risk as well as a commercial one.
CAWS R to Y vs Uniclass Ss and Pr
CAWS puts building services in groups R to X, with group Y as the shared engineering reference. Uniclass 2015 describes the same work as systems (Ss) that contain products (Pr), which suits MEP design and maps more cleanly to BIM objects and asset data.
A well-structured CAWS services specification keeps system requirements in R to X and refers to Y, rather than repeating the same pipework clause in five places.
NBS states that CAWS is no longer maintained as a classification system and that more projects are choosing Uniclass. CAWS is still widely used and readable by every UK contractor, but new section content and classification updates now happen in Uniclass. Uniclass 2015 is a suite of 12 tables maintained by NBS with an independent advisory board, free to download and updated regularly; NBS issued quarterly updates in January and April 2026. NBS's own small works building services content is offered as a Uniclass library rather than a CAWS one.
An architectural specification in CAWS and a services specification in Uniclass is workable, but agree it at the outset and make sure cross-references between the two documents use the right codes. Our guide to CAWS vs Uniclass covers the trade-offs.
Should building services be specified by performance or by product?
Most UK MEP specifications mix the two, and the choice should follow who carries the design risk. Specify by performance when the contractor completes the design; specify by product when there is a reason a particular item must be used.
By performance
The contractor completes the design. State the outcome and the standards, then what must be submitted to prove compliance: calculations, selections, commissioning results.
By product
The client has standards to follow, a product meets a planning condition or energy strategy, or it is part of a tested assembly. Name it and say whether substitutions are allowed.
The failure is a hybrid that does neither job: a performance clause that also names a product without saying which governs. If the product cannot meet the stated performance, the contract contains a contradiction.
A performance clause that also names a product needs one more sentence: which of the two governs.
Which regulations and standards does it need to cite?
Cite current editions and check them at each issue. In October 2026 the key moving items are Approved Documents L and F and the IET Wiring Regulations.
- 24.03.2026
ADL and ADF published
Future Homes and Buildings Standards, with the building circular.
- 15.10.2026
BS 7671 A2+A3 withdrawn
Cite BS 7671:2018+A4:2026 instead.
- 24.03.2027
ADL and ADF in force
Transitional arrangements for work already notified.
- 24.09.2027
In force for HRB work
Later date for higher-risk building work.
Because the new Approved Documents L and F come into force during many current projects, the specification should state the edition the design follows. Electrical clauses citing BS 7671:2018+A2:2022+A3:2024 need updating.
Other Approved Documents touch services too: B (fire, including fire stopping of service penetrations and fire detection), G (sanitation and hot water safety) and P (electrical safety in dwellings). See our post on energy performance specifications for Part L in more detail.
Withdrawn or superseded standards are a measurable problem. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.
How do you coordinate MEP with the architectural specification?
Coordination failures are where MEP specifications cost the most. Check six interfaces explicitly before every issue, and make sure each deliverable is owned by exactly one document.
- Penetrations and fire stopping
- Who specifies and installs fire stopping around services through compartment walls and floors? One document, one product system, matching the fire strategy.
- Builders' work (BWIC)
- Holes, plinths, supports and access panels are often missing from both specifications or duplicated in both.
- Sanitaryware and fittings
- Decide whether appliances sit in the architectural specification (CAWS N13 or the Uniclass equivalent) or the services one, and make sure the connections match.
- Luminaires, switches, sockets
- Often selected by the architect but specified by the engineer. Finishes, mounting heights and accessibility requirements must agree.
- Ceilings, risers, plant spaces
- Ceiling void depths, access hatches and plant room acoustics must agree across both documents and the drawings.
- Controls, commissioning, handover
- Commissioning, witnessing, O&M manuals and asset data are needed for practical completion and, on higher-risk buildings, the golden thread.
Drawing coordination matters as much as document coordination. The same Avoice report found drawing-to-spec mismatches in 71% of documents at contract issue, the one defect class that grew as projects moved from Stage 4 to tender to contract.
What are the most common gaps in MEP specifications?
Most gaps fall into five patterns. Each can be checked for before issue.
- Design responsibility is stated. Nothing is specified by performance without saying who completes the design.
- No open TBCs on priced items. Plant capacities, equipment selections and control strategies are not left "to be confirmed" when the contractor prices.
- No leftover template content, such as a sprinkler section on a building with no sprinklers.
- No orphan references. "Refer to the services drawings" only where that drawing is in the issued set.
- Testing and commissioning have acceptance criteria, so practical completion can be agreed.
Where Avoice fits
Avoice launched MEP specification writing in October 2026, so mechanical, electrical and plumbing sections can be drafted in the same workspace as the architectural specification, in CAWS or Uniclass.
Because both documents sit together, Avoice's QA/QC checks can flag contradictions and broken cross-references between them, and between the specification and the drawings, before issue.
Software can draft, cross-check and flag. The services engineer and architect still decide the design and who is responsible for it.
Write MEP and architectural specs in one place
Draft services sections in CAWS or Uniclass alongside the architectural specification, then check both against each other and the drawings before issue.
This guide is for general education and is not professional advice. Check current editions of Approved Documents and standards, and agree design responsibilities for your project, before relying on any clause.