In 2026 London, “it looks fine” is no longer a structural strategy.
London projects are full of legacy reinforced concrete frames — office blocks being converted, podiums being re-cut,
basements being reworked, façades being re-anchored, and old slabs being asked to carry new loads.
In 2026, structural investigation is no longer a nice-to-have. It is an evidence package that directly affects programme, cost,
dutyholder exposure and the credibility of your Gateway submissions.
The shift is simple: the regulator does not assess intent, it assesses evidence.
If your project sits inside BSR jurisdiction — or is moving into it —
transferring building control functions to the BSR becomes a regulatory reset,
and any legacy assumptions about structure are likely to be challenged.
This matters most at
Gateway 2, where the BSR expects materially complete, coordinated design.
For legacy frames, “coordinated design” means your structural model, strengthening strategy, fixings and interfaces are supported by
investigation outcomes — not guessed from old drawings.
1. What “good” looks like in 2026: investigation as a design input, not a report
Investigation is no longer a post-design validation exercise. In 2026 London projects, it is a design input that determines whether your
proposed interventions are feasible, buildable and approvable.
For a legacy concrete frame, the investigation should be capable of answering five questions:
- What is actually there? Member sizes, reinforcement layout, cover, thickness, support conditions.
- What condition is it in? Corrosion risk, carbonation depth, chloride presence, cracking mechanisms, previous repairs.
- What is the realistic capacity? Not theoretical — based on verified geometry/material parameters and load paths.
- What are the critical interfaces? Façade anchors, new openings, transfer beams, mezzanines, plant loads, stair cores.
- What changes create regulatory risk? Anything that could trigger design uncertainty, change-control events or rework later.
This is why competence expectations on dutyholders matter. If your structural strategy relies on assumptions,
it becomes a programme threat — and in 2026 the BSR is increasingly linking quality of evidence to dutyholder credibility.
That is directly connected to
competence expectations for Principal Designers.
2. The investigation stack the market expects (NDT + intrusive + lab)
Legacy concrete frames in London rarely behave like the drawings. A robust investigation stack typically combines:
Non-destructive testing (NDT) to map and de-risk
- Cover meter / rebar locator: reinforcement position and cover variability.
- GPR (where suitable): slab/beam scanning for rebar density and anomalies.
- UPV / rebound hammer (carefully interpreted): uniformity screening (not a strength certificate).
- Crack mapping + distortion surveys: identify structural mechanisms vs cosmetic cracking.
Intrusive investigations to confirm reality
- Breakouts: confirm bar size, spacing, laps, anchorage, cover, local defects.
- Core samples: compressive strength testing and density checks (and petrography if needed).
- Proof of support conditions: bearings, continuity, slab edges, hidden beams, stitch joints.
- As-built verification around proposed openings: trimming steel, local thickening, embedded services.
Laboratory and durability testing (where risk is present)
- Carbonation depth: predicts corrosion initiation risk on older frames.
- Chloride testing: critical for basements, car parks, de-icing exposure, historic leakage zones.
- Petrography: when unusual degradation mechanisms are suspected.
The outcome should not be a “data dump”. It should produce design-ready parameters and constraints that can feed the structural model,
strengthening details, and change-control plan.
3. The London failure points: why investigations go wrong (and cost you months)
London projects fail on investigation not because teams do nothing, but because they do the wrong thing at the wrong time.
These are the most common 2026 failure points:
- Too late: investigations scheduled after design decisions are “locked”, forcing re-design when reality emerges.
- Too light-touch: NDT used as a substitute for intrusive verification on critical interfaces.
- Access illusions: live buildings, out-of-hours rules, listed constraints, tenant interfaces not planned early.
- Uncontrolled breakouts: poor sequencing creates damage risk and dispute with landlords/occupiers.
- Missing lab lead-times: cores taken but results not available when design needs them (programme falls into a hole).
In 2026, these failures translate into regulatory consequences at submission stage, because
Gateway 2 does not reward “we will confirm later”.
If your structural evidence is incomplete, the design is not considered frozen — and that becomes a rejection risk.
4. Interfaces that now trigger liability exposure
Legacy frames are rarely isolated. They connect to façades, fire strategy, fixings, temporary works, and occupation plans.
In 2026, liability exposure grows fastest where investigation gaps exist at interfaces.
Façade anchors, brackets and fixings
- Anchor capacity depends on actual concrete quality, cover, edge distance and reinforcement conflict.
- Unknown rebar patterns create a workmanship trap: drill hits steel, relocations happen, performance drifts.
- Fire performance and accountability are now linked to evidencing what you actually build.
This links directly to the emerging reality that
façade systems are now treated as regulated safety systems with liability exposure.
If your fixing strategy is based on assumptions, you create a compliance risk and a future dispute risk.
New openings, structural alterations and change-of-use loads
- Cutting openings changes load paths and can expose hidden beams, stitch joints or weak bearing zones.
- Change-of-use often introduces higher imposed loads, vibration criteria, and new service penetrations.
- Strengthening choices (FRP, steelwork, concrete repairs) must be grounded in verified geometry and material parameters.
5. A 2026 investigation checklist (what to have before you “freeze”)
Use this checklist to validate whether your investigation package is strong enough to support design freeze and submission confidence:
Scope & Strategy
✔ Clear objective: capacity verification, durability risk, fixings strategy, alterations, or all of the above.
✔ Investigation locations mapped to proposed interventions and interfaces (not random “coverage”).
✔ Access plan confirmed (tenants, OOH, heritage/consents, noise/dust controls).
✔ Investigation locations mapped to proposed interventions and interfaces (not random “coverage”).
✔ Access plan confirmed (tenants, OOH, heritage/consents, noise/dust controls).
NDT + Intrusive
✔ Rebar mapping completed where drilling/fixings/openings are planned.
✔ Breakouts confirm bar sizes/spacing and support conditions at critical nodes.
✔ Core testing planned with lab lead-times aligned to design milestones.
✔ Breakouts confirm bar sizes/spacing and support conditions at critical nodes.
✔ Core testing planned with lab lead-times aligned to design milestones.
Durability & Condition
✔ Carbonation/chloride risk assessed where exposure history suggests corrosion potential.
✔ Crack mapping links symptoms to mechanisms (not just photographs).
✔ Repairs history identified and tested where relevant (unknown patch repairs are common in London stock).
✔ Crack mapping links symptoms to mechanisms (not just photographs).
✔ Repairs history identified and tested where relevant (unknown patch repairs are common in London stock).
Evidence & Accountability
✔ Design parameters extracted: thickness, reinforcement assumptions verified, material values evidenced.
✔ Findings captured into the project evidence trail (Golden Thread logic).
✔ Dutyholder sign-off route is clear and aligns with Principal Designer competence expectations.
✔ Findings captured into the project evidence trail (Golden Thread logic).
✔ Dutyholder sign-off route is clear and aligns with Principal Designer competence expectations.
6. Programme reality: investigations influence occupation strategy too
If your project involves partial completion, live interfaces, or early handover, investigation evidence affects more than design.
Weak structural certainty can undermine commissioning logic, interface management and occupation safety cases.
This connects directly with
phased occupation and partial Gateway 3 approval expectations.
Key takeaway
In 2026 London, legacy concrete frames must be treated as an evidence problem before they are treated as a design problem.
If you can’t prove what’s there, you can’t freeze design. If you can’t freeze design, you expose programme and submissions.
And if you can’t evidence interfaces (fixings, openings, repairs), you create long-tail liability that survives completion.
image: constructionmagazine.uk
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Expert Verification & Authorship: Mihai Chelmus
Founder of London Construction Magazine | Construction Testing & Investigation Specialist | 15+ years in construction, 10+ years delivering projects in London. Writing practical guidance on regulation, compliance and real on-site delivery reality.
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