UK Government Physical Security Guidelines

Physical security remains a front-line defense against intrusion in government buildings. The UK’s NPSA (formerly CPNI) outlines strict guidance for doors and lighting to eliminate tool-based attacks and visibility gaps that threaten facility integrity

UK Door Security Standards: Why Door Gap Sizes Matter

Tight Door-to-Frame Clearance

• Aim for minimal clearance between the door leaf and its frame—typically around 2 mm at the sides and top. This is the recommended gap in UK standards to ensure a secure, properly fitted door.

Deep Rebates and Overlapping Edges

• Use door frames with deep rebates (the recessed step the door closes against). Deeper, wider rebates help support the door edges and make it more difficult for an intruder to insert tools such as crowbars or wedges.

Well-Fitted Thresholds

• Keep the under-door gap as small as possible—ideally no more than about 5–6 mm. Larger gaps allow attackers to slide tools beneath the door. Security advisors suggest installing kick plates, door sweeps, or drop seals to reduce any space at the threshold.

Proper Hardware to Cover Gaps

• Fit latch guards or protective plates on outward-opening doors to cover the gap around the latch area. This prevents thin implements from slipping in to manipulate the latch. Hinge bolts (also known as dog bolts) on the hinge side are equally important, to stop the door being wedged open if there is any gap there.

How to Prevent Tool-Based Door Bypass

Keeping door gaps to a minimum is crucial because larger openings significantly increase the risk of unauthorised entry. UK security documentation often compares “small gap” installations—which are more secure—to installations with a “large gap,” which can be a clear weakness. Some common exploit methods include:

Prying and Forcing

• Where there is a noticeable gap—enough for a screwdriver or crowbar—an attacker can lever the door open. A larger gap also reduces the effectiveness of a deadbolt, as the bolt may not fully engage. Current security standards recommend minimal clearance alongside a long bolt throw for maximum resistance.

Latch Bypassing (“Shimming”)

• For latch-based locks, any gap near the latch side can allow thin plastic or metal shims to reach the latch bolt. Outward-opening doors without a latch guard are particularly vulnerable. Ensuring virtually no space for a tool to reach the latch (or using deadlocking latch bolts) is vital. Official guidance emphasises that a tight door fit makes it much harder to wedge any tool into the gap.

Under-Door Tools

• If the gap at the bottom of the door is large enough (often above about 6 mm), an attacker can slide an under-door tool inside to pull down an internal lever handle or manipulate a thumb-turn. Installing a drop bar or brush seal that leaves only minimal clearance helps prevent this.

Double Door Gaps

• Where two doors meet without an overlapping strip or astragal, the centre gap can be exploited. Attackers may slide objects through this gap to operate internal crash bars or locks. A rebated meeting stile or security astragal is strongly recommended to fill the gap and block these tools.

Door Gap Recommendations

According to UK government guidelines, clearances around all sides of a secure door should be kept to an absolute minimum—ideally 2–3 mm. Robust frame rebates and added hardware (such as latch guards and astragals) further help to cover any unavoidable gaps. Any gap exceeding roughly 5 mm is considered a security weakness that may allow intruders to insert tools. Ensuring snug door fits and installing gap-blocking features are straightforward yet vital steps in preventing forced or surreptitious entry.

UK Perimeter Lighting Requirements:  Why “Dim” Lighting is a Threat

Appropriate perimeter lighting is essential both to deter intruders and to support effective surveillance at night. UK security authorities, including the Center for the Protection of National Infrastructure (now NPSA), provide guidance on acceptable lighting levels—usually measured in lux—to avoid “dim” areas where intruders can conceal themselves. Key considerations include what qualifies as “dim” lighting, recommended lux levels, and how to ensure even coverage for CCTV.

What is “Dim” Lighting and Why It’s a Threat

In security terms, “dim” lighting refers to illumination that is below the levels needed for guards or cameras to reliably detect and recognize an intruder. Rather than a strict technical definition, it’s a qualitative description of insufficient light. According to UK government lighting guidance, at approximately 1 lux (one lumen per square meter), a person can just barely be detected against a background if they move, but details are indistinct npsa.gov.uk. This 1 lux level is extremely low – essentially moonlight levels – and is considered very dim for security purposes.

For effective security monitoring, lighting needs to be higher than this bare minimum. Research cited by the UK’s security experts found that around 3 lux of illumination (vertical, on a person) was required for even basic identification tasks under certain light types. In fact, a Home Office Forcible Attack Working Group study in the 1980s determined that roughly 1 lux was the threshold of detection and concluded that 3 lux should be the minimum “operational” lighting level for security at UK government sites. Lighting below ~3 lux would therefore be considered “dim” in a security context, as it approaches that detection threshold rather than providing a comfortable margin for observation. 

To put these numbers in perspective: 3 lux is still a relatively low light level (for comparison, a street-lit sidewalk might be 5–10 lux, and a typical office is ~300 lux). An area at 1–2 lux will appear very dark to the human eye once acclimated, meaning an intruder could be missed or only seen as a silhouette. Thus, any outdoor security area averaging only a couple lux or less is effectively dimly lit. Security guidelines use terms like “uniformity” and minimum lux levels to ensure there are no pockets of dim lighting (deep shadows or unlit corners) that could hide a person . In summary, dim lighting in security means illumination so low that detection is marginal – generally, lighting under a few lux falls into this category, whereas well-designed security lighting aims to stay at or above a recommended minimum (with allowances for maintenance fade) to avoid dim conditions. 

Meeting the Minimum Lux Levels for CCTV and Guard Visibility

UK official guidance provides clear recommendations for lighting levels around perimeters, fence lines, and gate/entry points to ensure visibility and effective CCTV performance at night. The goals are to eliminate deep shadows, provide even coverage, and allow security cameras to capture clear images. Key recommendations include:

Summary: Avoiding a ‘Dim’ Perimeter

A poorly lit perimeter is a clear liability. UK government standards advise maintaining at least 3 lux of uniform illumination across all protective areas, with higher levels at entrances. This ensures that there are no shadows or dark corners where an intruder could hide. In practice, new lighting systems are often designed to exceed 3 lux slightly—around 5 lux—to account for gradual lamp dimming over time. Entrances and gate areas are usually much brighter, often between 50 and 150 lux, to facilitate clear identification and security checks. By adhering to these guidelines, facilities can establish a well-lit perimeter that supports effective CCTV monitoring and deters intruders.

References:

• Centre for the Protection of National Infrastructure (CPNI/NPSA) guidance on physical security of doors and lighting (official UK government guidance).SECURITY LIGHTING - Guidance for Security Managers
• British Standards for door and lighting installations (e.g., BS 8214 for fire door gaps, BS 5489 for lighting design).