SAP Calculations &
Area Weighted U-values
For Extensions to Dwellings
There are 3 Levels of SAP Assessment for Extensions
We offer three assessment routes to Part L compliance depending on your extension’s design and what Building Control require.
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1. The 25% Glazing Method
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2. The Area Weighted U-Value Calculation Method
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3. The Whole Dwelling SAP Calculation Method​​​
Accredited Assessors
Local - Nationwide Coverage
Prices From only £95
EPCs & SAP Calculations
Speedy Turnaround
How to pass Part L requirements for your extension
Planning a beautiful new extension often means lots of light and glazing — but it also means meeting certain energy requirements.
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Under Part L of the Building Regulations, extensions must be assessed for energy performance to ensure they meet modern standards. This is done through SAP calculations, which are required for Building Control approval.
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There are three recognised routes to proving compliance, depending on your glazing and what Building Control request. Below is a simple guide to help you choose the right method.
There are 3 Ways to Prove an Extension Complies with Part L:
Building control allow for multiple ways to prove compliance. As well as meeting the u-value requirements for all the fabric elements you can prove compliance by:
NEW ARRIVALS
This method ensures the total area of windows, roof windows, rooflights and doors in extensions do not exceed the sum of the following.
i. 25% of the floor area of the extension.
ii. The total area of any windows and doors which no longer exist or are no longer exposed due to the extension.
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Energy Ready can calculate and provide confirmation of compliance with a SAP Calculation for Building Control.
This assessment ensures the total area of windows, roof windows, rooflights and doors in extensions do not exceed the sum of the following.
i. 25% of the floor area of the extension.
ii. The total area of any windows and doors which no longer exist or are no longer exposed due to the extension.
​
Energy Ready can calculate and provide confirmation of compliance with a SAP Calculation for Building Control.
When an extension has more glazing than the 25% rule allows, compliance can still be achieved using an area weighted U-value calculation.​
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We assess the U-values of all new elements — walls, roof, floor, windows and doors — and combine them into one overall U-value. This is then compared to a notional extension built to the minimum standards in Approved Document L.
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If your overall U-value is lower, the extension passes. If it’s higher, we’ll advise what upgrades are needed to achieve compliance.
** If individual U-value Calculations are required we can complete these for £65 Each**
** If individual U-value Calculations are required we can complete these for £65 Each**
This method provides the most detailed route to compliance. We assess the existing dwelling and the new extension together, then compare the combined performance against a notional dwelling built to the minimum fabric and energy standards in Approved Document L.
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If the whole dwelling performs as well as or better than the notional version, the extension passes Part L compliance.
If not, we advise the upgrades—typically insulation, glazing, or heating efficiency—that will bring the project into compliance.
This approach is usually required for:
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Complex designs
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Extensions with very high glazing
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Projects where Building Control request a “Full SAP”
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Homes where earlier methods fail to achieve compliance
What we'll need to get started
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Floor plans (existing + proposed)
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Elevations
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Section showing construction details (if available)
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Window/door schedule
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Any guidance from Building Control
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U-Value certificates if available
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Energy Ready can build u-values if required at £65 each
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Hover for more detail
An Accurate SAP Calculation Improves Your Extension's Value and Comfort
An energy-efficient extension not only meets legal requirements but also enhances the comfort and value of your home. A well-insulated extension with an efficient heating system will stay warmer in the winter and cooler in the summer, making it a more comfortable living space. Additionally, a higher energy performance calculation for an extension can lower energy bills, making your home more attractive to future buyers.
We specialise in providing detailed and accurate Thermal Performance Assessments for Extensions, helping you achieve an optimal SAP rating. By identifying areas where insulation, heating, or other energy systems can be improved, we ensure that your extension is both energy-efficient and cost-effective.
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What Does a SAP Calculation for an Extension Involve?
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A SAP calculation for an extension looks at several factors to determine how energy-efficient your extension will be. The calculation considers elements such as:
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The U-values of walls, roofs, and windows, which measure how well these elements retain heat.
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The type of heating system installed in the extension.
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How much natural light the extension receives (affecting heating and lighting needs).
For large extensions or those with a significant thermal performance impact, a SAP calculation will ensure that your extension doesn't negatively affect the overall energy performance of your home. A SAP Rating for Residential Extension is essential for demonstrating compliance with Part L and obtaining a completion certificate from Building Control.​​​​
Put your money into your extension NOT energy bills!
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Straight Forward SAP Calculations
For Extensions
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Dedicated, fully qualified assessor
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Unlimited recalculations to get the building to a pass
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Expert knowledge - fully accredited
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Experienced in navigating SAP Calculation Building Regulations
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All documentation provided for Building Control sign off​
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3 day turnaround upon receiving all information
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SAP Calculations for Extensions across the UK
Find Us All Over East Anglia and Beyond!
Ipswich
Woodbridge
Felixstowe
Colchester
Chelmsford
Thetford
Norwich
Bury St Edmunds
Cambridge
Benefits of a SAP Calculation for an extension with Energy Ready?
Working with Energy Ready has several advantages for builders, architects, and developers. Here are the key benefits of SAP calculations in the UK:
Ensures Regulatory Compliance
Ensures your new build design complies with SAP Calculation Extension UK building regulations. All our assessors are fully qualified.
Improves
Sustainability
Promotes sustainable building practices by evaluating the environmental impact.
Better Building
Energy Efficiency
Ensures the new build meets energy efficiency standards, reducing energy consumption and costs for your end customer
Enhances
Property Value
Boosts property marketability and value by ensuring high energy performance. A high rating is an attractive selling point to potential buyers or renters.
Identify Cost Saving Opportunities
Identifies cost-saving opportunities in the design phase by giving you insights to optimise energy use.
Forecast your 'As Built' Energy Efficiency
Provides precise predictions on energy performance, aiding in better decision-making during construction.
How to pass compliance for my extension
Level 1
25% Glazing Allowance Method
This method ensures the total area of windows, roof windows, rooflights and doors in extensions do not exceed the sum of the following.
i. 25% of the floor area of the extension.
ii. The total area of any windows and doors which no longer exist or are no longer exposed due to the extension.​
Energy Ready can calculate and provide confirmation of compliance with a SAP Calculation for Building Control.
Level 2
Area Weighted U-Value Calculation Method
When an extension has more glazing than the 25% rule allows, compliance can still be achieved using an area weighted U-value calculation.​​We assess the U-values of all new elements — walls, roof, floor, windows and doors — and combine them into one overall U-value. This is then compared to a notional extension built to the minimum standards in Approved Document L.​
If your overall U-value is lower, the extension passes. If it’s higher, we’ll advise what upgrades are needed to achieve compliance.
​
** If individual U-value Calculations are required we can complete these for £65 Each**
Level 3
Whole Dwelling SAP Assessment Method
This method provides the most detailed route to compliance. We assess the existing dwelling and the new extension together, then compare the combined performance against a notional dwelling built to the minimum fabric and energy standards in Approved Document L.​
If the whole dwelling performs as well as or better than the notional version, the extension passes Part L compliance. If not, we advise the upgrades—typically insulation, glazing, or heating efficiency—that will bring the project into compliance.
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This approach is usually required for:​
-
Complex designs
-
Extensions with very high glazing
-
Projects where Building Control request a “Full SAP”
-
Homes where earlier methods fail to achieve compliance
Our promise to you...
Speedy Turnaround
When we receive the go ahead you become our priority. We aim to turnaround within 3 days of receiving all the plans.
Competitive Prices
We review all our prices regularly so you get the very best value for your money and with FREE as designed EPCs for every SAP calculation of an extension we are hard to beat!
Transparent Process
No hidden fees, no sudden invoices, no confusing jargon, no extra waiting time. We work with you to get the result you need in line with SAP calculations for Building Regulations Part L.
Open & Easy Dialogue
Our goal is to help you achieve yours. From start to finish no question is off limits, we want you to be completely satisfied and confident in the energy efficiency of your project after working with us.
Get even more with the Energy Ready blog pages....
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