Confined Space Risk Assessment: A UK Step-by-Step Guide

A confined space risk assessment is the process of identifying the hazards inside an enclosed space, judging how serious they are, and deciding the controls needed before anyone enters. Under UK law you must first ask whether entry can be avoided altogether — and only if it cannot does the assessment move on to how to make entry safe. The assessment is what your confined space permit to work is built on: no valid assessment, no safe permit.

This guide covers UK requirements and references HSE guidance. It does not constitute legal advice and is not a substitute for site-specific assessment by a competent person.

What counts as a confined space?

A confined space is not defined by size. The Confined Spaces Regulations 1997 define it as a place that, "by virtue of its enclosed nature," gives rise to a "reasonably foreseeable specified risk" (such as serious injury from a hazardous atmosphere, drowning, fire, or asphyxiation). Two things together, in other words: it is enclosed, and that enclosure creates one of the specified risks. A large open tank can be a confined space; a small cupboard usually is not. Common examples include tanks, vessels, silos, sewers, manholes, pits, ductwork, and unventilated rooms where gases can collect.

If you are still mapping out which of your tasks need formal control, our permit-to-work system guide covers how confined space entry fits alongside hot work, working at height, and other high-risk activities.

The law: avoid entry first

The Confined Spaces Regulations 1997 set the order of priorities, and it is not the order most people expect. Regulation 4 says no one should enter a confined space to carry out work "unless it is not reasonably practicable to achieve that purpose without such entry." In plain terms: the first question your risk assessment must answer is do we need to go in at all?

A surprising amount of confined space work can be done from outside — using long-reach tools, remote cameras, jetting equipment, or by emptying and cleaning a vessel in place. If the assessment can eliminate entry, the most dangerous hazards disappear with it.

Only where entry is genuinely unavoidable does Regulation 4(2) require a safe system of work that renders the work safe so far as is reasonably practicable. That safe system is documented through a confined space permit and the risk assessment behind it. The duty to assess in the first place comes from Regulation 3 of the Management of Health and Safety at Work Regulations 1999, which requires a "suitable and sufficient" risk assessment.

The five steps of a confined space risk assessment

The HSE risk assessment framework — identify hazards, assess the risks, control the risks, record your findings, review — applies to confined spaces with the specific atmospheric and access hazards layered on top.

Step 1: Identify the hazards

Confined space hazards fall into a few recurring groups:

• Oxygen deficiency — oxygen displaced by other gases, or consumed by rusting steel, rotting organic matter, or curing concrete. The normal safe range is roughly 19.5% to 23% oxygen.
• Oxygen enrichment — leaking oxygen (for example from cutting equipment) raises fire and explosion risk dramatically.
• Toxic atmospheres — hydrogen sulphide in sewers, carbon monoxide from engines, solvent vapours from cleaning or coating.
• Flammable atmospheres — residues, vapours, or gas that can ignite. The standard action threshold is 10% of the lower explosive limit (LEL).
• Engulfment and drowning — free-flowing solids (grain, sand) or liquids that can flood the space.
• Physical and access hazards — restricted entry, moving machinery, extreme temperature, poor lighting.

The space's history matters as much as its current state. What was last stored in it? What process runs next to it? A vessel that is "empty" can still hold sludge, vapour, or trapped liquid.

Step 2: Assess the risks

For each hazard, judge how likely harm is and how severe it would be, taking into account the number of workers, how long the work will take, and the conditions on the day. A two-minute inspection and a four-hour cleaning job in the same tank carry very different risk profiles. This step is where you decide which hazards need engineering controls, which need atmospheric monitoring, and what the entry conditions must be.

Step 3: Control the risks

Controls follow the hierarchy — eliminate first (avoid entry), then reduce. Typical confined space controls include:

• Isolation — lock off feeds, agitators, and energy sources that could start, fill, or move while someone is inside
• Cleaning and purging — remove residues; ventilate or purge to make the atmosphere safe before entry
• Atmospheric testing and monitoring — test before entry and monitor continuously inside, for oxygen, flammable gas, and the specific toxic gases identified in Step 1
• Ventilation — forced-air ventilation to maintain a safe atmosphere during the work
• Safe access and communication — a reliable way in and out, and a way to stay in contact with the person inside
• Emergency and rescue arrangements — Regulation 5 of the Confined Spaces Regulations 1997 requires suitable emergency arrangements before entry. Rescue cannot be improvised on the day, and "we'll call 999" is not a rescue plan.

Step 4: Record the findings

Document the hazards, the risk judgement, and the controls. This record feeds straight into the permit. Anyone authorising or entering the space should be able to read the assessment and understand exactly what the dangers are and what has been done about them.

Step 5: Review

A confined space risk assessment is not a one-off. Review it when anything relevant changes — new equipment, a different substance, a change in the team — and periodically to confirm it still holds. Conditions inside an enclosed space can change between shifts.

From risk assessment to permit

The risk assessment and the permit are two halves of the same control. The assessment decides what the dangers are and what controls are needed; the permit confirms, at the moment of entry, that those controls are actually in place and authorises the specific entry. Our confined space permit template guide walks through the permit document section by section — atmospheric testing results, the pre-entry checklist, personnel, authorisation, and closure.

If you want a quick pre-entry checklist to work from on site, our free safety checklist generator builds one based on your selected work types, including confined space entry.

Common mistakes

Assessing entry without asking whether entry is needed. Skipping straight to "how do we make entry safe" misses the most important control the law actually requires first.

Testing once and assuming it stays safe. Atmospheres change. Disturbing sludge, running a process nearby, or simply time passing can re-introduce gas. Continuous monitoring during the work is the norm for any space with an atmospheric hazard.

No rescue plan, or one that relies on the emergency services. The Confined Spaces Regulations require suitable emergency arrangements in place before entry. Most confined space fatalities include would-be rescuers who entered without protection.

Generic, copied assessments. A risk assessment for "confined spaces" in general is not suitable and sufficient. Each space and each task needs its own assessment grounded in that space's hazards and history.

Key references

• Confined Spaces Regulations 1997 — the core duties, including avoiding entry (reg 4) and emergency arrangements (reg 5)
• L101 — Safe work in confined spaces (HSE) — the Approved Code of Practice and guidance for the 1997 Regulations
• Confined spaces: a brief guide (INDG258) (HSE) — plain-English overview
• Management of Health and Safety at Work Regulations 1999, Regulation 3 — the duty to assess risk

For how confined space entry sits alongside other permit types, see our types of permit to work guide.