From 2025 onwards, material substitutions on regulated projects are no longer a commercial decision. They are a regulated change. Under the Building Safety Act, the Building Safety Regulator (BSR) treats any change affecting fire safety, structure, or building performance as a controlled design variation. This applies regardless of whether the substitution is cheaper, faster, more available, or equivalent in the contractor’s view.
In 2026, BSR enforcement is fully operational. Inspectors are auditing live construction sites, checking Golden Thread records and blocking Gateway approvals where unapproved substitutions are identified.
The old site logic of we’ve always used this alternative no longer applies. If the building is regulated, the design is locked at Gateway 2. Any deviation must follow formal change control.
In 2026, BSR enforcement is fully operational. Inspectors are auditing live construction sites, checking Golden Thread records and blocking Gateway approvals where unapproved substitutions are identified.
The old site logic of we’ve always used this alternative no longer applies. If the building is regulated, the design is locked at Gateway 2. Any deviation must follow formal change control.
What Counts as a Material Substitution Under BSR
BSR does not define substitution by procurement logic. It defines substitution by risk impact.
A substitution is regulated if it affects:
Typical regulated substitutions include:
If it appears in the fire strategy, structural design, or compliance drawings, it is regulated. If it contributes to the building’s safety case, it is regulated.
BSR does not define substitution by procurement logic. It defines substitution by risk impact.
A substitution is regulated if it affects:
- Fire performance
- Structural behaviour
- Durability
- Load paths
- Acoustic separation
- Thermal performance
- Moisture control
- Compartmentation
- Smoke control
- Escape routes
Typical regulated substitutions include:
- Fire-stopping products
- Cladding systems
- Insulation types
- Structural fixings and anchors
- Balustrades and balconies
- Facade interfaces
- Roofing systems
- Load-bearing elements
- Waterproofing systems
If it appears in the fire strategy, structural design, or compliance drawings, it is regulated. If it contributes to the building’s safety case, it is regulated.
What the BSR Explicitly Allows
BSR does not ban substitutions, it bans unapproved substitutions. Material changes are allowed only when processed through formal design change control.
The following conditions must be met:
If the change does not affect safety-critical systems and is proven technically equivalent, it may be classified as a non-notifiable change, but it must still be recorded. BSR enforcement teams are auditing change logs on live projects. If it is not in the log, it is treated as unauthorised.
BSR does not ban substitutions, it bans unapproved substitutions. Material changes are allowed only when processed through formal design change control.
The following conditions must be met:
- Change formally proposed by the Principal Contractor
- Impact assessment produced by the Principal Designer
- Fire engineer sign-off where applicable
- Structural engineer sign-off where applicable
- Updated drawings issued
- Golden Thread updated
- Change submitted to BSR where notifiable
- Acceptance recorded before installation
If the change does not affect safety-critical systems and is proven technically equivalent, it may be classified as a non-notifiable change, but it must still be recorded. BSR enforcement teams are auditing change logs on live projects. If it is not in the log, it is treated as unauthorised.
What the BSR Prohibits
BSR prohibits:
BSR prohibits:
- Installing alternative products without design approval
- Value engineering safety-critical systems without review
- Changing tested fire systems without retesting
- Altering facade build-ups without reassessment
- Substituting fixings without structural verification
- Swapping insulation systems without fire re-certification
- Changing cavity barrier systems
- Using similar products without performance evidence
Common failures now triggering enforcement:
Under BSR, equivalent is not a valid justification without revalidation.
- Different fire stopping brand same rating
- Alternative insulation with equivalent lambda
- Different anchor system same load capacity
- Changed waterproofing membrane same spec
- New facade supplier same classification
Under BSR, equivalent is not a valid justification without revalidation.
Gateway Control
Once Gateway 2 is approved, the design is frozen.
Any substitution that affects:
Once Gateway 2 is approved, the design is frozen.
Any substitution that affects:
- Fire strategy
- Structure
- External wall system
- Means of escape
- Compartmentation
...requires formal change control.
Unapproved substitutions discovered during inspections can trigger:
Gateway 3 (Occupation Approval)
At Gateway 3, BSR reviews the as-built record.
Inspectors check:
If substitutions exist without approval:
This is now happening on live schemes.
- Stop notices
- Compliance notices
- Construction suspension
- Gateway reset
- Refusal of Gateway 3
Gateway 3 (Occupation Approval)
At Gateway 3, BSR reviews the as-built record.
Inspectors check:
- Installed materials vs approved design
- Fire-stopping records
- Installation evidence
- Product certification
- Test evidence
- Inspection records
- Change control log
If substitutions exist without approval:
- Occupation can be blocked
- Completion certificates withheld
- Remedial works ordered
- Criminal enforcement initiated
This is now happening on live schemes.
The Golden Thread
The Golden Thread is now a live compliance record, not a handover file.
It must contain:
BSR inspectors are cross-checking:
Mismatch = enforcement.
The Golden Thread is now a live compliance record, not a handover file.
It must contain:
- Approved product specifications
- Installation records
- Inspection photos
- Test certificates
- Change approvals
- Compliance declarations
BSR inspectors are cross-checking:
- Design drawings
- Procurement schedules
- Delivery notes
- Installed materials
- Test evidence
Mismatch = enforcement.
Legal Responsibility — Who Is Accountable
Under the Act, liability is personal.
Accountable persons include:
Site managers, package managers and supervisors can be individually liable if they knowingly install unapproved substitutions.
This includes:
Under the Act, liability is personal.
Accountable persons include:
- Client
- Principal Designer
- Principal Contractor
Site managers, package managers and supervisors can be individually liable if they knowingly install unapproved substitutions.
This includes:
- Criminal prosecution
- Unlimited fines
- Professional sanctions
- Director disqualification
Commercial pressure is not a defence.
Programme pressure is not a defence.
Supply chain issues is not a defence.
What a Compliant Substitution Process Looks Like in 2026
A compliant substitution now follows this chain:
No approval = no installation.
A compliant substitution now follows this chain:
- Substitution identified
- Design impact assessed
- Fire engineer review
- Structural engineer review
- Specification update
- Drawing update
- Golden Thread update
- BSR notification (if required)
- Formal approval
- Installation
- Inspection
- Record retention
No approval = no installation.
The Commercial Reality
Contractors are discovering that:
The commercial model of informal site substitution is no longer viable on regulated buildings.
Conclusion — The End of Informal Site Decisions
The Building Safety Regulator has fundamentally changed site authority. Material substitutions are no longer controlled by procurement, they are controlled by law. In 2026, compliance is binary: Either the change is approved or it is illegal.
Contractors are discovering that:
- Late substitutions delay Gateway approvals
- Delays cost more than original materials
- Remedial works erase savings
- Enforcement risk is existential
- Insurance exposure is rising
- PI insurers are tightening
The commercial model of informal site substitution is no longer viable on regulated buildings.
Conclusion — The End of Informal Site Decisions
The Building Safety Regulator has fundamentally changed site authority. Material substitutions are no longer controlled by procurement, they are controlled by law. In 2026, compliance is binary: Either the change is approved or it is illegal.
Projects that adapt their site governance early will continue to build. Projects that treat substitutions casually will face enforcement, the industry has entered the compliance era.
Image © London Construction Magazine Limited
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Expert Verification & Authorship: Mihai Chelmus
Founder, London Construction Magazine | Construction Testing & Investigation Specialist |
