Escape Window Regulations Explained

What Size Does an Escape Window Need to Be?

Where the relevant Approved Document B emergency escape-window guidance applies, the window should provide an unobstructed openable area of at least 0.33m².

In addition:

• The opening should be at least 450mm high.
• The opening should be at least 450mm wide.
• The bottom of the openable area should be no more than 1100mm above floor level.
• The window should be capable of remaining open without somebody having to hold it.

The important word is:

OPENABLE.

Building Control isn’t interested simply in the overall frame dimensions.

The 450 × 450 Mistake

This causes considerable confusion.

Some people hear:

“An escape window needs to be 450mm × 450mm.”

That’s wrong.

450mm × 450mm gives an area of:

0.2025m²

That’s substantially less than the required 0.33m² openable area.

You need to satisfy:

Minimum 450mm width + minimum 450mm height + minimum 0.33m² area.

What Does 0.33m² Mean?

It means the unobstructed area actually available for a person to escape through when the window is fully opened.

It is not:

• Overall frame size
• Glass size
• Sash size
• Brick opening size

The frame, sash and hardware reduce the available clear opening.

Example

Suppose the clear opening measures:

600mm × 550mm

0.600 × 0.550 = 0.33m²

That reaches the 0.33m² area and both dimensions exceed 450mm.

But this is only a simple rectangular illustration.

Real window hardware and sash geometry can affect the unobstructed route.

Why Overall Window Size Is Misleading

Consider two casement windows with identical overall dimensions.

Window A uses standard friction stays.

Window B uses egress hinges designed to move the sash further clear of the opening.

Their escape openings can differ substantially.

This is why an installer shouldn’t promise escape compliance based solely on overall frame size.

Egress Hinges

Egress hinges or friction stays are designed to increase the usable clear opening from a casement window.

They are particularly useful where a window needs to provide emergency escape.

Tell the fabricator:

“This is an escape window.”

Do that before manufacture.

They can then select the appropriate:

• Sash
• Hardware
• Hinge
• Opening configuration

Side-Hung vs Top-Hung

A side-hung casement often provides a more practical escape arrangement than a small top-hung vent.

However, compliance depends on the actual clear opening.

Don’t rely solely on opening style.

Measure the resulting unobstructed area.

Where Are Escape Windows Required?

This depends on the building’s fire strategy and layout.

Approved Document B provides emergency escape-window guidance for particular dwelling situations, including relevant rooms at lower levels.

Do not publish or tell customers that:

“Every bedroom legally needs an escape window.”

That is too broad.

The requirements depend on:

• Storey
• Building layout
• Protected escape routes
• Room arrangement
• Fire strategy
• Applicable Building Regulations guidance

Inner Rooms

Escape windows can also become relevant where an inner room is permitted under the applicable Approved Document B provisions.

This is another reason the requirement can’t be determined simply by asking:

“Is it a bedroom?”

Look at the whole layout.

Maximum Height Above Floor

Where the cited escape-window guidance applies, the bottom of the openable area should be no more than 1100mm above the floor.

Note the wording:

Bottom of the openable area.

Don’t automatically measure to the external cill, frame or glass line.

Finished Floor Level

As with safety glazing, measure from the finished floor level.

On a building site, the floor may subsequently receive:

• Insulation
• Screed
• Timber
• Tiles
• Carpet

Make sure the design uses the correct final level.

Locks

The Approved Document allows locks, including locks with removable keys, in the relevant escape-window provision.

The important consideration is that the complete escape arrangement remains appropriate.

Child-Resistant Release Catches

Opening stays with child-resistant release catches may also be used under the relevant guidance.

This is important where Part B and protection-from-falling considerations overlap.

Restrictors

This requires careful thought.

A window may simultaneously need to address:

• Emergency escape
• Falling risk
• Child safety
• Purge ventilation
• Overheating ventilation

Don’t install a restrictor simply because:

“It’s upstairs.”

The selected arrangement needs to satisfy the relevant requirements together.

Escape Windows and Part K

Part K may require consideration of protection from falling.

Part B may require an escape opening.

The solution should address both.

Possible hardware solutions may allow normal restricted operation with a release mechanism providing the required opening when necessary.

Use suitable products and follow the design requirements.

Escape Windows and Part O

On new residential buildings, opening windows may also contribute to the overheating strategy.

A restrictor or reduced opening angle can therefore affect more than fire escape.

Coordinate the requirements.

Escape Windows and Security

Security is another consideration.

Part Q may apply to accessible windows in new dwellings.

The window therefore potentially needs to provide both:

• Appropriate security
• Required emergency escape function

Use a tested/certified configuration appropriate to the project.

Replacement Windows

When replacing a window that forms part of the building’s means of escape, do not inadvertently make the escape provision worse.

For example:

Existing window:

• Large side-hung opening
• Good clear opening

Replacement:

• Smaller opening sash
• Standard hinges
• Reduced clear opening

The replacement may look better and have a lower U-value but could create a fire-safety issue.

Loft Conversions

Escape provision in loft conversions needs to be considered as part of the complete fire strategy.

Don’t simply assume that fitting a large roof window or casement automatically resolves Building Regulations.

Follow the applicable design and Building Control requirements.

Common Escape Window Mistakes

Mistake 1 – Using overall frame size

Wrong measurement.

Mistake 2 – Thinking 450 × 450mm is enough

It isn’t.

Mistake 3 – Forgetting the 0.33m² area requirement

All relevant dimensions need to be satisfied.

Mistake 4 – Measuring to the wrong point

Use the unobstructed openable area.

Mistake 5 – Using standard hinges

They may reduce the clear opening.

Mistake 6 – Adding a restrictor afterwards

It could compromise the intended escape function.

Mistake 7 – Making the replacement opening smaller than the existing escape provision

Always identify escape windows at survey stage.

Real-World Example

A bedroom window has an overall frame size of:

900mm × 1050mm

The builder assumes this is obviously large enough for escape.

However, the design has:

• Fixed pane
• Narrow side-opening sash
• Standard friction hinges

The actual clear opening is insufficient.

The problem isn’t the overall window size.

It’s the opening configuration.

Changing the design before manufacture could solve the problem easily.

Changing it after installation is considerably more expensive.

Survey Checklist

Mark every potential escape window clearly on the survey.

Record:

• Overall opening size
• Finished floor level
• Bottom of openable area
• Required clear opening
• Opening style
• Hinge type
• Restrictor requirements
• Part K considerations
• Part Q requirements where applicable

Then put:

EGRESS / ESCAPE WINDOW

clearly on the order.

Frequently Asked Questions

What is the minimum escape-window opening?

Where the relevant Approved Document B provision applies, the unobstructed openable area should be at least 0.33m², with both height and width at least 450mm.

Does 450 × 450mm comply?

No. Its area is only 0.2025m².

What is the maximum height from the floor?

The bottom of the openable area should be no more than 1100mm above floor level under the relevant guidance.

Is that measured from finished floor?

Yes, use the relevant finished floor level.

Does every bedroom require an escape window?

No. It depends on the building layout and escape strategy.

Do replacement windows need escape openings?

Where an existing window provides required escape provision, the replacement design needs to account for that function.

What are egress hinges?

Hardware designed to help maximise the clear opening available from the sash.

Can escape windows have locks?

The Approved Document’s relevant provision permits locks.

Can escape windows have restrictors?

Opening stays with child-resistant release catches can be used in the relevant guidance, but the complete arrangement must preserve the necessary escape provision.

Related Guides

• Part B Explained
• Part K Explained
• Window Restrictors Explained
• PAS 24 Explained
• New-Build Window Regulations
• Building Control Checklist

Trade Tech Technical Support

When requesting a quotation, clearly identify every window intended to provide emergency escape.

We can then consider sash configuration and suitable hardware before manufacture.

Important: Escape provision is part of the building’s overall fire strategy. The dimensions in this guide relate to the specific Approved Document B guidance where emergency escape windows are used and should not be treated as a universal rule for every building.