From Feasibility to Specification: What Happens at RIBA Stage 4 Concrete Specification?

TL;DR: RIBA Stage 3 (Spatial Coordination) establishes feasibility and outline intent; RIBA Stage 4 (Technical Design) is where bespoke concrete moves from outline specification to manufacturing-ready detail – dimensions, fixings, finishes and tolerances all locked down. Getting this transition right protects your design intent, avoids costly late changes, and keeps your Stage 4 concrete specification aligned with BS 8500 and Eurocode 2 from day one.

Every bespoke concrete element you specify – a feature wall panel, a cantilevered stair tread, a reception desk cast to your exact brief – has to travel the same journey: from an idea sketched into a feasibility study, through to a manufacturing drawing approved to the last millimetre. That journey has a name in the RIBA Plan of Work: the move from Stage 3, Spatial Coordination, to Stage 4, Technical Design.

For architects and specifiers working with a bespoke concrete manufacturer, this transition is where design ambition meets production reality. Get it right, and your concrete elements go into manufacture with confidence, on programme and without surprises. Get it wrong, and you risk delayed approvals, redesigned moulds and decisions that are expensive – sometimes impossible – to reverse.

This guide walks through what changes between the two stages, what MASS needs from your team to keep things moving, and where the genuine pinch points sit before manufacturing approval.

What changes between RIBA Stage 3 and Stage 4 for bespoke concrete?

According to the RIBA Plan of Work, Stage 3 is formally known as Spatial Coordination, while Stage 4 is Technical Design. The distinction isn’t just semantic – it defines the deliverables, the level of detail, and the decisions that become locked in.

Stage 3 deliverables are about establishing that a design works spatially: outline drawings, early coordination between disciplines, and an outline specification that sets direction without fixing every detail.

Stage 4 is where that outline becomes a technical document. According to NBS guidance on specification across the project timeline, an updated outline specification is expected at the end of Spatial Coordination, with the final specification only complete once all design tasks are finished at the end of Technical Design. As specification lead Patrick Crocock puts it, the specification is “the key document that defines the quality of the product, the quality of execution and the required work at completion.”

For bespoke concrete, that means Stage 3 drawings might show a panel’s approximate dimensions and intended finish, while Stage 4 drawings specify exact tolerances, fixing centres, joint widths and sealant detail – the information our workshop actually manufactures from. The practical impact on your programme is significant: Stage 4 decisions, once drawn and approved, set the mould, the production sequence and the delivery date.

What information does MASS need from architects at Stage 3?

Feasibility is only as strong as the information behind it. At Stage 3, we ask your design team for:

• Spatial envelope and approximate dimensions – even rough sizing lets us flag weight, handling and transport constraints early.
• Intended finish and texture direction – board-mark, polished, exposed aggregate or a bespoke colour match all demand different production routes.
• Structural interface assumptions – how the concrete element meets the building structure, even in outline.
• Programme milestones – when Stage 4 drawings are expected, and when manufacturing needs to start to hit site dates.

Missing or vague information at this stage doesn’t just slow us down – it tends to resurface as a late, costly problem at Stage 4, once drawings are further along and changes are harder to absorb. An outline specification that names a finish class and a rough tolerance band is worth far more to us than a beautiful render with no technical framing behind it.

What must be resolved at Stage 4 before manufacturing approval?

Technical Design is where ambiguity has to end. Before we can issue manufacturing approval, the following need to be resolved:

• Dimensions and tolerances – final, to-the-millimetre sizing, including allowances for thermal movement and construction tolerance.
• Fixings – type, location and load path, agreed with the structural engineer.
• Finishes – confirmed texture, colour and surface treatment, matched against an approved sample.
• Design freeze – the point at which no further changes are accepted without triggering a change-control process.

This is also where coordination with other trades becomes critical, which we’ll come to below. In a design freeze, every discipline agrees the model and drawings are final – not “nearly final.” For a bespoke concrete element, that freeze triggers mould design, which is where real production cost begins to accrue.

Coordinating concrete details with structure, joinery and M&E

Bespoke concrete rarely exists in isolation. A reception desk needs power and data routed through it; a stair tread needs to interface with balustrade fixings; a wall panel needs to clear a sprinkler head or a lighting track. The CIC BIM Protocol is clear that clash detection and model coordination remain the responsibility of the design lead, not any single trade contractor – which means concrete coordination has to be actively managed, not assumed.

At Stage 4, this typically means:

• Structural engineers confirming load paths and fixing capacities
• Joinery or M&E consultants confirming service penetrations, cable routes and junction details
• A shared model (or coordinated drawing set) showing the concrete element alongside its neighbouring elements, checked for clashes before drawings are issued for manufacture

The earlier this coordination happens, the less it costs. A clash discovered in CAD is a drawing revision. The same clash discovered after casting is a replacement panel.

How minor design changes affect moulds, supports and costs

Here’s an illustrative example, not a specific project: imagine a reception desk design where, late in Stage 4, the client asks for a service penetration to move 150mm to accommodate a revised cable route. On paper, that’s a small change. In production, it can mean:

• A revised or rebuilt mould, since the cutout location is often cast into the formwork itself
• Reassessment of internal reinforcement, which may now clash with the new opening
• A delay to the production slot originally booked for that panel, with knock-on effects for delivery

Published research on design changes during production confirms this isn’t unique to concrete – peer-reviewed evidence shows design changes made after production has started generate measurable redesign and construction-change costs across construction more broadly. The lesson for bespoke concrete is specific: the closer a change sits to design freeze, the more of the production chain it disturbs.

Which decisions become costly to reverse after drawing approval?

Some decisions lock in cost and programme the moment drawings are approved:

• Overall dimensions and cast geometry – the mould is built to these; changing them means rebuilding it.
• Fixing type and location – reinforcement and cast-in fixings are positioned to match; late changes often mean starting the cast again.
• Colour and aggregate mix – once a batch is matched and approved against sample, switching mid-production risks visible inconsistency across the piece.
• Finish class – moving from a honed to a highly polished finish, for example, changes the refinement process and can affect programme significantly.

Treat drawing approval as a genuine commitment point, not a formality – it protects your design intent as much as it protects the production schedule.

Three missing inputs that most often delay manufacturing approval

In our experience, the same three gaps recur most often:

1. Unresolved fixing design. Without structural sign-off on fixing type and loading, we can’t finalise reinforcement or cast-in details.
2. No approved sample reference. Colour, texture and aggregate exposure need a physical benchmark, not just a written description – verbal colour specifications almost always lead to dispute later.
3. Incomplete coordination with adjacent trades. Service penetrations, structural interfaces or joinery junctions confirmed “later” routinely resurface as changes after drawings are issued.

Resolving these three before requesting manufacturing approval is the single most effective way to protect your programme.

Stage 4 concrete specification and sign-off checklist

Before signing off a Stage 4 concrete specification, check the following:

• Final dimensions and tolerances confirmed, including allowance for movement
• Fixing type, location and structural capacity agreed and documented
• Finish, texture and colour matched against an approved physical sample
• Concrete specified in line with BS 8500, the complementary British Standard to BS EN 206, with exposure class and consistence appropriate to application
• Structural design referenced against Eurocode 2 (EN 1992)
• Coordination checked against structural, joinery and M&E drawings or model, with no outstanding clashes
• Design freeze communicated to all parties, with a clear change-control process for anything after
• Manufacturing programme and delivery date confirmed against site sequencing

Explore MASS’s Stage 2–3 Feasibility Pack

The strongest Stage 4 concrete specification starts with strong groundwork at Stage 3. If you’re approaching feasibility on a project involving bespoke concrete – wall panels, floor panels, stair treads, signage, surfaces, furniture or sinks – explore MASS’s Stage 2–3 Feasibility Pack to understand what we need from your team early, and how to carry that information cleanly through to technical design.

Speak to our team on 01202 628 140 to discuss your project and the information we’ll need to move confidently from feasibility to specification.

Frequently Asked Questions

What’s the difference between RIBA Stage 3 and Stage 4 for concrete specification?
Stage 3 (Spatial Coordination) establishes feasibility with outline dimensions and intent. Stage 4 (Technical Design) fixes final dimensions, fixings, finishes and tolerances, producing the manufacturing-ready specification.

How long does it take to go from feasibility to manufacturing approval?
Timelines vary by project complexity, but incomplete fixing design, missing sample approval, or unresolved trade coordination are the most common causes of delay – resolving these early keeps the process on track.

What standards govern a Stage 4 concrete specification?
In the UK, BS 8500 (the complementary standard to BS EN 206) governs constituent materials and specification approach, while Eurocode 2 (EN 1992) governs structural design.

What happens if a design changes after drawing approval?
Changes after design freeze typically require mould revisions, reinforcement reassessment, or re-casting, with associated cost and programme impact – which is why drawing approval should be treated as a firm commitment point.

Who is responsible for coordinating concrete details with other trades?
Clash detection and model coordination sit with the design lead, though structural, joinery and M&E input all feed into the final Stage 4 drawings for a bespoke concrete element.