When Is Air Tightness Testing Required in the UK?
If you’re building or converting a property in the UK, you may be legally required to carry out air tightness testing to comply with Building Regulations Part L. This test plays a vital role in measuring how energy-efficient your building is by checking for unwanted air leakage. In this article, we explain when air tightness testing is needed, what the rules are for different types of projects, and why getting it right is essential for your SAP Calculations, EPC rating, and overall compliance.
🏡 What Is Air Tightness Testing?
Air tightness testing (also called a blower door test) measures how much air leaks through gaps, cracks, and unsealed openings in a building’s structure. It’s a key part of assessing the energy efficiency of a property and is often required to comply with UK Building Regulations Part L.
At Assessment Hive, we provide fast, reliable air tightness testing across the UK, helping you meet compliance and improve energy performance.
âś… When Is Air Tightness Testing Legally Required?
1. New Builds (Single or Multiple Dwellings)
Air tightness testing is mandatory for most new homes under Approved Document Part L. It verifies that the building achieves the required air permeability rate (usually 5 m³/(h·m²) @ 50 Pa or less).
Your result directly affects your:
If you’re building a single new dwelling, it must usually be tested individually unless certain exemptions apply (see below).
2. Developments with Multiple Units
When constructing multiple similar homes (e.g., a housing estate), air tightness testing can be done on a sample basis:
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A set percentage of homes must be tested
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All homes in the sample group must be identical in design and construction
⚠️ If one property fails, more testing or remedial work will be required across the development.
3. Extensions, Conversions & Material Change of Use
Air tightness testing may be required for:
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Loft conversions
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Extensions that significantly increase envelope area
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Barn conversions or any material change of use
While not always mandatory, testing is often required when:
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The project creates a new dwelling unit
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SAP calculations are needed to demonstrate compliance
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Requested by your building control officer
Not sure if your project needs a test? → Get in tocuh
⚖️ What Determines If Testing Is Needed?
| Factor | Details |
|---|---|
| Type of project | New builds almost always need testing. |
| Number of dwellings | Sampling allowed in multi-unit sites. |
| Air permeability target | If ≤ 5 m³/(h·m²), testing is essential. |
| SAP Calculations | Required for all new dwellings and many conversions. |
| EPC rating | The test result directly impacts your EPC score. |
đźš« Are There Any Exemptions?
Yes — testing may not be required if:
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The development is small (e.g., one or two units)
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A default air permeability value (15 m³/(h·m²)) is used in SAP
However:
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Using the default value can negatively impact your SAP Calculations and EPC
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This may lead to failing Part L or require upgrades to insulation, windows, etc.
So while technically exempt, it’s often not advisable to skip testing.
🛠️ When Should the Test Be Carried Out?
Air tightness testing should be done:
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After the building envelope is complete (windows, doors, roof, walls)
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Before applying internal finishes like plaster or carpets
This ensures accurate results and allows time to address any leaks.
đź’¬ Summary: When You Need Air Tightness Testing
| Type of Project | Is Testing Required? |
|---|---|
| New build (single home) | âś… Yes |
| New development (2+ homes) | âś… Yes (with sampling) |
| Barn conversion | 🔶 Sometimes |
| Loft conversion or extension | 🔶 Sometimes |
| Commercial building | âś… Often required |
🔧 Book Your Air Tightness Test with Assessment Hive
At Assessment Hive, we make the Air Tightness Test process straightforward. Our team works with homeowners, architects, and builders nationwide to provide quick, accurate assessments and practical advice to achieve compliance.
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 Call us on 020 7183 3240
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