The RIBA Plan of Work does not officially contain standalone sub-stages titled "Stage
4A" or "Stage 4B."
In the official RIBA Plan of Work (2020 Overview & Detailed Guide), there is only Stage 4:
Technical Design (covered on pages 34–37 of the RIBA Plan of Work 2020 Overview). RIBA
explicitly notes that when elements are procured via a Contractor Designed Portion (CDP),
the technical design is split between the lead design team and specialist subcontractors.
However, RIBA intentionally leaves the exact division of duties to the project’s contract
documents and Design Responsibility Matrix (DRM) rather than publishing a fixed,
universal checklist of façade specification clauses.
The industry documents and exact sections that define the division of responsibilities
between Stage 4a (Design Intent / Performance Specification) and Stage 4b (Specialist
Detailed Design / Manufacture) are outlined below.
1. CWCT Guide to the Procurement of Systemised Building Envelopes
•
The Document: Published by the Centre for Window and Cladding Technology
(CWCT, University of Bath). This is the definitive UK contractual guide for envelope
procurement.
•
Exact Sections & Pages:
o
Section 2: Allocation of Design Responsibility (Pages 8–11): Establishes
the boundary between the Designer (lead consultant/façade engineer) and
the Cladding Contractor. Page 9 states that the Designer is responsible for
establishing all design criteria (actions, movements, performance classes,
aesthetic constraints), while the Cladding Contractor is responsible for the
design of components, connections, and fabrication details.
o
Section 3.3: Tender Design / Performance Specification (Pages 12–14):
Lists what the designer must provide at tender stage (Stage 4a). It explicitly
states that the specification must give complete performance parameters
(design wind pressures, barrier loads, thermal transmittance targets, CWCT
weathertightness classes, acoustic ratings), so that tenderers can select and
price a system without making unverified design assumptions.
o
Section 3.4: Detailed Design by the Cladding Contractor (Pages 15–18):
Corresponds to Stage 4b, explicitly assigning structural member
calculations, bracket sizing, anchor verification, profile extrusion drawings,
and workshop fabrication tolerances to the specialist subcontractor.