Independent guidance · Survey guides
Four-Stage Clearance
The UKAS-accredited clearance process that certifies an enclosure is safe to reoccupy after licensed asbestos removal — every stage explained.
Key takeaways
- Stage 1 — Preliminary check of site condition
- Stage 2 — Thorough visual inspection
- Stage 3 — Air monitoring
- Stage 4 — Final assessment post-enclosure
- Independence and competence
Four-stage clearance is the HSG248 process by which an independent, UKAS-accredited analyst certifies that an enclosure used for licensed asbestos removal is safe to hand back to the client. It is required after every licensed removal, is not optional, and must be carried out by an analyst independent of the removal contractor. This guide sets out what each of the four stages involves and where projects most commonly fall down.
Stage 1 — Preliminary check of site condition
Before the enclosure is dismantled, the analyst reviews the site paperwork (plan of work, notification, waste consignment), inspects the enclosure integrity, checks the negative pressure unit and confirms the extraction has been running for the required period. This is the earliest opportunity to stop a bad clearance from happening — a compromised enclosure at Stage 1 means the clearance stops until it is corrected.
What this means
Stage 1 catches paperwork and enclosure failures before anyone breathes anywhere near the space.
Stage 2 — Thorough visual inspection
The analyst enters the enclosure in appropriate PPE and RPE and carries out a systematic visual inspection of every surface — floors, walls, ceiling, plant, framework, edges, penetrations, ledges. Any residual dust, debris or fibrous material triggers a fail and a return to cleaning. The inspection is documented photographically and against a written checklist. This is the stage where marginal removals get caught.
What this means
Visually clean is the standard. Not 'nearly clean', not 'clean enough' — visually clean under torchlight.
Stage 3 — Air monitoring
Reassurance air monitoring is carried out inside the enclosure while the enclosure is subject to disturbance — typically brushing surfaces and directing the air movement device around the space. Fibre concentrations are measured by phase-contrast microscopy (PCM) to MDHS 39/4 with a clearance indicator of below 0.01 fibres per cm³ at each sampling point. Any exceedance triggers a fail and a return to cleaning.
What this means
Air monitoring under disturbance conditions is the stage that catches invisible residues. Below 0.01 f/cm³ or the enclosure stays up.
Stage 4 — Final assessment post-enclosure
Once the enclosure is dismantled, the analyst carries out a final visual assessment of the surrounding area and issues a Certificate of Reoccupation. The certificate names the analyst, the analytical laboratory (must be UKAS 17025 accredited for asbestos analysis), the site, the works cleared, and the date and time from which the area is safe to reoccupy. Without a Certificate of Reoccupation the client is not entitled to allow anyone back into the space.
What this means
The Certificate of Reoccupation is the legal handover document. No certificate, no reoccupation.
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Independence and competence
The analyst must be independent of the removal contractor. Analysts must be qualified to BOHS P403 (asbestos fibre counting) and P404 (air sampling and clearance) as a minimum, with P405 preferred for lead analysts. The analytical body must be UKAS 17025 accredited for the specific tests used. Any deviation from independence or accreditation invalidates the clearance evidentially.
What this means
Independent, P403/P404 qualified, UKAS 17025 accredited body. Three non-negotiables.
Common failure modes
The recurring failures we see are: analyst not booked in time and clearance slips programme; contractor rushing Stage 2 cleaning and failing on visual; TVOCs or dust from other trades contaminating Stage 3 sampling; enclosure dismantled before Stage 4 assessment completes; certificate issued without a UKAS-accredited laboratory reference. Each of these is avoidable and each has cost projects days or weeks.
What this means
Book the analyst when the removal contractor is booked, not the day before completion.
Frequently asked questions
Who carries out four-stage clearance?
A UKAS 17025 accredited analytical body, using analysts qualified to BOHS P403/P404, independent of the removal contractor.
Is four-stage clearance required after non-licensed removal?
A four-stage clearance is required after all licensed work. Non-licensed and NNLW work do not always require four-stage clearance but should be followed by a documented visual inspection and, for NNLW, appropriate reassurance monitoring.
What is the clearance limit?
Below 0.01 fibres per cm³ measured by PCM under disturbance conditions, at every sampling point.
What if the enclosure fails a stage?
The clearance stops, the reason is documented, remedial work (usually further cleaning) is carried out, and the failed stage is repeated. Certificates are only issued when all four stages pass.
How long does a four-stage clearance take?
Typical single-enclosure clearance is 2–4 hours on site plus analytical turnaround for any samples taken. Larger phased removals run rolling clearances room by room.
Can the removal contractor also do the clearance?
No. Independence is a fundamental requirement of HSG248 and any accredited analytical process. The clearance analyst must be from a separate organisation.
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