Digital and BIM
Stage 5 contractor design
MEP design for contractors and design-and-build appointments: Stage 4 taken to installation information, or the contractor design portion produced outright.
- Discipline
- Digital and BIM
- Typical timescale
- Against the construction programme — information issued in the order the site requires it
WHAT IT IS
Stage 5 is manufacturing and construction. The design work in it is the step between a coordinated Stage 4 design and information a subcontractor can order materials and install from: circuit-level electrical design, duct and pipe sizing carried through to the final route, plant selections against real manufacturers' data, builders work, and the drawings that go on site.
Two appointments are common. One is a contractor's design portion, where the main contract makes the contractor responsible for designing part of the MEP installation and the design capability has to come from somewhere. The other is a design-and-build project where the contractor has taken on the whole MEP design after novation or from a performance specification.
Both are the same engineering problem from a different side of the table: the design has to be buildable, procurable at the price that was tendered, and demonstrably compliant, and the deadline is a construction programme rather than a planning date.
WHEN YOU NEED IT
When a contractor holds design responsibility under a design-and-build or contractor's design portion arrangement and does not have the in-house discipline cover to discharge it.
When a scheme was tendered on a performance specification and now needs a design behind it before anything can be ordered.
When a Stage 4 design has been inherited — from a consultant no longer appointed, or from a novation — and it has to be checked, completed and taken to installation information under someone's professional responsibility.
And where an installation is already under way and a change on site needs designing and documenting properly rather than being resolved verbally and discovered at handover.
OUR PROCESS
The design responsibility matrix comes first, because on a Stage 5 appointment the most expensive ambiguity is not technical. BSRIA BG 6 sets out the mechanism: every system, and every element within it, is assigned to a party and a stage before the work starts. Where the matrix is silent, it is agreed and written down rather than assumed.
The inherited design is then checked rather than accepted. Anything being carried forward — load calculations, distribution sizing, plant selections — is verified against its own basis, because Stage 5 information issued on an unchecked assumption becomes an installed defect.
Design then proceeds to installation level: final circuit design and protective device selection to BS 7671, ductwork and pipework sized on the coordinated route rather than the schematic, plant selected against manufacturers' performance data with the duty confirmed, and builders work openings and structural loadings issued to the parties who need them.
Information is issued against the construction programme, in the order the site needs it, with a register showing what has been issued, what is due and what is being held pending a decision.
Designer duties under CDM 2015 apply here as they do at any other stage — the residual risks that could not be designed out are recorded and issued with the information, not left to the site to discover.
DELIVERABLES
What you receive
- Design responsibility matrix agreed before work starts
- Check report on any inherited design being carried forward
- Installation-level drawings against the construction programme
- Final circuit design, protective device and cable schedules
- Plant selections with manufacturers performance data and confirmed duties
- Builders work drawings and structural loading information
- Information issue register with dates and outstanding decisions
- Residual risk register under CDM 2015
STANDARDS
Standards applied
The editions in force at the time of design. Where a standard is amended mid-project we confirm which edition the appointment is based on.
- RIBA Plan of Work 2020Stage 5 — Manufacturing and ConstructionRIBA
- BSRIA BG 6/2018A Design Framework for Building Services — design responsibility matrixBSRIA
- BS 7671:2018+A4:2026Requirements for Electrical InstallationsIET
- CDM 2015Construction (Design and Management) Regulations 2015 — designer duties
- BSRIA BG 49/2015Commissioning Building ServicesBSRIA
- BS EN ISO 19650-2:2018Delivery phase of the assetsBSI
QUESTIONS
Common questions
What is a contractor design portion?
A contractor design portion, or CDP, is a part of the works that the building contract makes the contractor responsible for designing, while the rest of the design stays with the client team. In MEP work it commonly covers specialist systems such as sprinklers, lifts, or a specific plant package. The contractor carries the design liability for that portion and normally procures the design capability to discharge it.
What is the difference between Stage 4 and Stage 5 design?
RIBA Stage 4 is technical design — the design is developed and coordinated to the point where it can be tendered and built from, and it is complete as a design. Stage 5 covers manufacturing and construction, where the remaining design work is what a subcontractor needs to order and install: final circuits, exact routes on the coordinated model, plant selected against a specific manufacturer, and builders work.
Can you take on a design produced by another consultant?
Yes, and this is common after novation or where a consultant appointment has ended. Anything being carried forward is checked against its own basis first — loads, distribution sizing and plant selections — because taking over a design means taking responsibility for it, and an unchecked assumption at Stage 5 becomes an installed defect. The check is reported before the design work continues.
Who holds the design liability on a design-and-build project?
The contractor holds it towards the client under the building contract, and the designer they appoint holds it towards the contractor under the consultancy appointment. What each is responsible for is set by the design responsibility matrix rather than by the contract title, which is why agreeing that matrix before the work starts matters more than any other single step.