Part L Building Regulations: energy efficiency guide for London homeowners

What Part L covers

Part L of the Building Regulations is titled "Conservation of fuel and power." It is the section of the Building Regulations that deals with energy efficiency -- how well a building retains heat, how efficiently it generates and distributes warmth, and how much carbon dioxide it produces as a result of its energy use.

Part L is divided into several volumes:

• Volume 1: Dwellings -- covers new homes, extensions to existing homes, conversions, and replacement of thermal elements (windows, roofs, external walls) in existing homes. This is the volume that affects most London homeowners.
• Volume 2: Buildings other than dwellings -- covers commercial, industrial, and public buildings.

For London homeowners planning an extension, loft conversion, or any other building work, Part L determines the minimum insulation levels, glazing performance, heating system efficiency, and airtightness that your project must achieve. Building control will not sign off your project unless it meets Part L standards.

Part L works alongside the other parts of the Building Regulations. While Part L deals with energy, other parts cover structure (Part A), fire safety (Part B), ventilation (Part F), overheating (Part O), and many more. All must be satisfied, but Part L is the one that has changed most significantly in recent years and has the greatest impact on construction specification and cost.

The 2021 uplift and the road to Future Homes Standard

Part L was substantially revised in June 2022 with what is commonly called the "2021 uplift" (the consultation was in 2021, the implementation in 2022). This uplift introduced significantly tighter U-value requirements, new ventilation standards (Part F), and for the first time, a requirement to address overheating (Part O).

The 2021 uplift was explicitly designed as a stepping stone towards the Future Homes Standard (FHS), which the government has said will come into force in 2025-2026. The FHS will require new homes to produce 75-80% less carbon emissions than 2013 standards. It is expected to effectively mandate heat pumps in new builds and require extremely high levels of insulation and airtightness.

What this means for London homeowners in 2026

If you are planning an extension, loft conversion, or renovation in 2026, the 2021 uplift standards are the current requirement. The full Future Homes Standard applies primarily to new-build dwellings. However, extensions and conversions are also affected because:

• The elemental U-value targets from the 2021 uplift already apply to all new work
• When you replace a thermal element (e.g. replace your roof covering, or strip and rebuild an external wall), you must upgrade the insulation to current Part L standards
• Any new or replacement windows must meet current U-value and solar gain requirements
• If your extension includes a new heating system or extends an existing one, the heating must meet current efficiency standards

The practical impact: if you are building a rear extension or converting your loft in London today, you need thicker insulation, better-performing windows, and more careful airtightness detailing than you would have needed five years ago. The standards are achievable and the costs are manageable, but your drawings and specification must account for them from the start.

U-value requirements explained

A U-value measures how effectively a building element (wall, roof, floor, window) prevents heat from passing through it. It is measured in W/m²K (watts per square metre per degree Kelvin). The lower the U-value, the better the insulation.

Here are the current Part L U-value targets for extensions and alterations to existing dwellings:

These are limiting values -- the worst your building elements can be. In practice, you may need to achieve better U-values in some elements to compensate for poorer performance elsewhere (the "trade-off" approach). Your building regulations drawings should specify the insulation type, thickness, and U-value for every element of the new construction.

How insulation thickness affects design

The tighter U-value requirements mean thicker construction. A wall that used to be 300mm thick may now need to be 380mm to accommodate the additional insulation. In a loft conversion, the rafters may need to be deepened or insulation added over the rafters, reducing available headroom. In a rear extension, the thicker walls reduce the internal floor area slightly.

These are all manageable with good design, but they need to be accounted for from the outset. Our building regulations drawings specify the full wall, roof, and floor build-ups with insulation thicknesses, ensuring the design achieves compliance without surprises on site.

SAP calculations: when you need one

SAP stands for Standard Assessment Procedure. It is the government's methodology for assessing the energy and environmental performance of dwellings. A SAP calculation produces an energy rating (the SAP score, from 1 to 100+) and determines the EPC (Energy Performance Certificate) rating.

When a SAP calculation is required

• New-build dwellings. A full SAP calculation (known as SAP 10.2, or the forthcoming SAP 11) is mandatory for all new homes. This includes new-build houses and flats, and conversions that create a new self-contained dwelling (e.g. converting a house into two flats).
• Conversions to create new dwellings. If your project involves creating a new self-contained dwelling from an existing building (change of use), a SAP calculation is required for the new dwelling.

When a SAP calculation is NOT required

• Extensions to existing dwellings. A standard rear extension, side extension, or loft conversion on an existing house does not require a full SAP calculation. Instead, you demonstrate compliance with Part L by meeting the individual elemental U-value targets.
• Replacement of thermal elements. When replacing a roof, external wall, or floor, you must upgrade to current U-value standards, but no SAP calculation is needed.

For most London homeowners extending or converting their loft, a SAP calculation is not required. The compliance route is straightforward: specify insulation, glazing, and heating that meet the elemental U-value and efficiency targets, and show this on the building regulations drawings.

Air permeability and airtightness

Air permeability measures how airtight a building is -- how much air leaks through the building envelope (walls, roof, floor, windows, and junctions). Draughty buildings waste enormous amounts of energy because heated air escapes through gaps, cracks, and poorly sealed junctions.

The current requirement

For new dwellings, an air permeability test (blower door test) is mandatory. The maximum allowable air leakage rate is 8 m³/h/m² at 50 Pascals, but the design target is typically 5 m³/h/m² or lower. Very well-built houses achieve 3 m³/h/m² or better. Passivhaus standard requires 0.6 air changes per hour at 50 Pascals, which is exceptionally airtight.

For extensions and loft conversions

An air permeability test is not normally required for extensions and loft conversions on existing dwellings. However, building control will expect to see that reasonable airtightness measures have been incorporated:

• Vapour control layer (VCL) on the warm side of insulation, with all joints taped
• Sealing around windows and doors with expanding foam and tape
• Sealed junctions between the new construction and the existing building
• Service penetrations sealed -- every pipe, cable, and duct that passes through the building envelope must be sealed

These details are shown on our building regulations drawings as construction section details, typically at 1:5 or 1:10 scale. Getting the airtightness right at the design stage prevents expensive remedial work on site.

Ventilation and MVHR

As buildings become more airtight, ventilation becomes more important. You cannot simply seal a building and hope for the best -- you need a controlled ventilation strategy to maintain air quality, remove moisture, and prevent condensation and mould.

Part F of the Building Regulations covers ventilation. For extensions and loft conversions, the main ventilation requirements are:

• Extract ventilation in kitchens (minimum 30 l/s intermittent or 13 l/s continuous), bathrooms (minimum 15 l/s intermittent or 8 l/s continuous), and utility rooms
• Background ventilation via trickle vents in windows (or continuous mechanical supply)
• Purge ventilation capability in habitable rooms (openable windows providing a ventilation opening of at least 1/20th of the floor area)

MVHR (Mechanical Ventilation with Heat Recovery)

MVHR systems extract stale, humid air from kitchens and bathrooms, pass it through a heat exchanger that recovers up to 90% of the heat, and supply fresh, filtered air to living rooms and bedrooms. MVHR is increasingly common in London extensions and loft conversions because:

• It maintains excellent indoor air quality without opening windows (important on busy London streets with noise and pollution)
• It recovers heat that would otherwise be lost through extract ventilation
• It reduces condensation and mould risk, which is particularly valuable in loft conversions where warm, moist air from below rises into the new rooms
• It is likely to become a de facto requirement under the Future Homes Standard

MVHR is not currently mandatory for extensions or loft conversions (simple extract fans and trickle vents can achieve compliance), but we increasingly recommend it for larger projects where the energy recovery benefits justify the upfront cost (£3,000 to £6,000 installed for a typical domestic system).

EPCs and the impact on property value

An Energy Performance Certificate (EPC) rates the energy efficiency of a dwelling from A (most efficient) to G (least efficient). The EPC is based on a SAP calculation and shows the property's predicted energy costs, carbon emissions, and recommendations for improvement.

When you need an EPC

• Selling or renting a property. You must have a valid EPC (less than 10 years old) before marketing a property for sale or rent.
• After building work that affects energy efficiency. If you build an extension, convert a loft, or create a new dwelling, a new EPC should be produced to reflect the improved thermal performance. Building control may require this as a condition of the completion certificate.

EPC and property value

Research consistently shows that properties with higher EPC ratings command higher sale prices and rental values. A 2024 study by the Department for Energy Security and Net Zero found that moving from an EPC rating of D to C adds approximately 3-5% to a property's value. Moving from D to B can add 6-14%.

When you build a Part L compliant extension or loft conversion, you are not just adding floor space -- you are also improving the overall thermal performance of the property, which can improve the EPC rating and add value beyond the raw square footage.

How Part L affects common London projects

Rear extensions

A standard single-storey rear extension must achieve the elemental U-values for walls (0.18), floor (0.18), roof (0.15), and glazing (1.4). The main design impact is wall thickness (typically 100mm+ of PIR insulation in a timber frame wall or full-fill cavity in masonry) and the flat roof build-up (150mm+ of rigid insulation in a warm roof construction). Bifold doors and large areas of glazing must be specified with care -- the total glazed area should not exceed 25% of the extension floor area under the default compliance method, or you need to demonstrate compliance via the trade-off route.

Loft conversions

For loft conversions, the main Part L challenge is the roof. Achieving a U-value of 0.15 at rafter level requires approximately 150-180mm of rigid insulation (PIR) between and over the rafters. This reduces available headroom, which is critical in London's Victorian and Edwardian terraces where roof pitches are often modest. The trade-off between insulation thickness and headroom needs careful design. Our loft conversion drawings specify the exact insulation build-up to achieve compliance while maximising headroom.

Mansard roof conversions

Mansard conversions effectively create a new external envelope (walls and roof), so Part L applies to every element. The near-vertical mansard walls must achieve a U-value of 0.18 and the mansard roof (the shallow-pitched top section) must achieve 0.15. Because the mansard creates a new roof structure from scratch, insulation is easier to integrate than in a dormer conversion where you are working within an existing roof.

Change of use conversions

When converting a building to residential use (e.g. office to flat, or shop to dwelling), Part L requires the thermal elements to be upgraded to current standards "as far as reasonably practicable." This means the external walls, roof, and floor must be insulated to current U-value targets where possible. A SAP calculation is also required for the new dwelling. These projects can be among the most challenging for Part L compliance because the existing building fabric may limit what insulation can be achieved.

Compliance routes

There are two main routes to demonstrating Part L compliance for extensions:

1. Elemental approach (simplest)

Meet each individual U-value target for walls, roof, floor, and glazing. Ensure the heating system serving the new space meets efficiency requirements. Limit the glazed area to 25% of the extension floor area. This is the most straightforward route and the one we use for most standard London extensions and loft conversions.

2. Whole-building approach (more flexible)

Demonstrate that the overall carbon emissions from the dwelling (including the extension) do not exceed a target emission rate. This approach allows you to trade off one element against another -- for example, you could have more glazing than 25% if you compensate with better wall and roof insulation or a more efficient heating system. This route requires a SAP calculation and is used for projects where the design cannot meet the individual elemental targets.

For most London homeowner projects, the elemental approach is simpler, cheaper, and perfectly adequate. We only recommend the whole-building approach when the design requires it -- typically for extensions with large areas of glazing or projects in heritage buildings where insulation options are constrained.

Cost implications

Part L compliance adds cost to your project, but the uplift is smaller than many homeowners fear.

For a typical London rear extension costing £50,000 to £80,000, the Part L uplift represents approximately 5-10% of the total construction cost. This is offset by reduced energy bills -- a well-insulated extension is significantly cheaper to heat than one built to older standards, and the payback period is typically 8-15 years.

Our building regulations drawings include full Part L specifications -- insulation types, thicknesses, U-value calculations, and construction details -- so your builder can price accurately and there are no ambiguities on site.

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Frequently asked questions