SAP Calculations for Extensions

(Heat Loss Calculations)

Since 2007, our highly experienced SAP Assessors have provided SAP calculations (heat loss calculations) for thousands of highly glazed residential extension projects with varying degrees of construction complexity and site limitations.

We have a 100% record in demonstrating compliance with Part L1B of the Building Regulations. We are therefore confident that you will not need to compromise your design if you instruct us to undertake the SAP calculations for your extension project.

Heat Loss Calculations for Highly Glazed Extensions – Expert Assessment & Advice to Guarantee Compliance





SAP Calculations are a useful way of demonstrating that highly glazed extensions, orangeries and garden rooms comply with the overall requirements of Part L1B of the Building Regulations via the ‘Design Flexibility Criterion’.

 Often referred to as heat loss calculations for over-glazed extensions, Building Control Bodies are increasingly requesting SAP calculations for extension projects as a result of the growing trend to incorporate high levels of glazing within designs including the bi-fold doors, floor to ceiling windows and roof lights and lanterns.

To discuss your specific project in greater detail, or to obtain a FREE quotation please give us a call on 0115 7270599, email us at, or complete our enquiry form.

Do I need a SAP Calculation for my extension?

Part L1B of the Building Regulations limits the amount of glazing within extensions to 25% of the new floor area (plus the areas of any existing windows that are removed as part of the works). This is to prevent excessive heat losses arising from the new extension owing to glass having a reduced thermal performance in comparison to the insulation standards required for external walls and roofs.

If a project meets the above allowance, then there is no mandatory requirement to provide a SAP calculation for the extension works.

Owing to current trends for light and airy internal spaces, many extensions designs readily exceed the 25% glazing allowance.

In such instances, Part L1B provides a ‘design flexibility’ option, which provides a mechanism for the applicant to build the proposed design if they can demonstrate that on completion, the overall dwelling will be no less efficient than if a conventionally glazed extension of the same dimensions had been constructed instead.

One method for demonstrating compliance with Part L1B via the design flexibility criterion is to provide the Building Control Body (BCB) with a SAP Calculation for the highly glazed extension.

SAP Calculations FAQs

How are SAP Calculations used to show that a highly glazed extension complies with Part L1B of the Building?

Two alternate SAP calculations are produced for the project.  The first is based on the existing property with a notional extension of the same dimensions as the proposed works with a standard level of glazing.

This ‘Notional’ SAP calculation provides a calculated level of annual carbon dioxide emissions that could theoretically be emitted from the development under Part L1B of the Building Regulations and provides a target threshold for the proposed works.

A second ‘Actual’ SAP calculation is then prepared for the existing house and the actual specification and glazing levels of the proposed extension. The actual calculated annual carbon dioxide emissions are then compared to the previously calculated target rate.

If the proposed extension carbon dioxide emissions are less than the target threshold, the extension is deemed to be compliant with Part L1B of the Building Regulations via the Design Flexibility Criterion without any additional requirements.

What if the SAP Calculation Fails?

If the proposed extension carbon dioxide emissions are higher than the calculated target threshold, trade-off improvements will be required to either the extension specification or the existing house as a means offsetting the additional heat loss associated with the extra levels of proposed glazing.

Commonly recommended improvements include increased levels of insulation to the extension floor, roof or walls, improved glazing specifications, top-up loft insulation or cavity wall insulation to the existing property or improvements to the current heating system.

When should the SAP Calculations be completed?

SAP Calculations for extensions should ideally be completed as soon as the architect or designer has finalised the layout and proposed build specification of the extension works.  This ensures that if the SAP calculations show an initial compliance failure, specification changes can be easily identified and made before works have commenced on site.

How can EPS Group help with SAP Calculations?

There are many reasons to appoint EPS Group to provide your SAP Calculations

How can EPS Group help with SAP Calculations?

At EPS Group, we are interested in forming ongoing partnerships with property professionals and self-builders by offering unrivalled customer support and technical consultation, which can be relied upon.
As part of this commitment, our SAP Calculation service includes all required advice and recommendations to ensure that your project passes with the appropriate Building Regulations in a practical and cost-effective manner.

Don’t let compliance obligations hold you back! Our Energy Consultants and certified Assessors have the experience to understand your challenges, and the expertise to meet them on your behalf.


Highly Experienced

We have been successfully providing SAP calculations for extensions since 2007 and have a 100% record in demonstrating compliance with Part L1B of the Building Regulations.


Competitive Pricing

We provide transparent and competitive fees with our extensive experience also providing an opportunity to reduce the build costs of your extension project.


Expedited Service Available

We appreciate that heat loss calculations can often be overlooked during the planning and construction of a home extension. If you require SAP Calculations for your extension urgently, pleased give us a call and we will try our best to help.


Independent Advice

We only provide energy compliance services. With no materials to sell or commissions to earn, you can be confident that we will aim to provide you with the most economic and feasible solutions to achieving compliance.


Client Focused

We tailor our services around you and your bespoke requirements whilst taking every opportunity to make the building regulations work in your favour.


Fully Accredited Team

Our Energy Assessors are fully accredited and insured. You can therefore rely on the advice and services that they provide.

EPS Group: A Partner for those in the Construction Industry

We value the importance of a solid working relationship with those in the construction industry, and for many, we serve as the reliable go-to-company to entrust from the planning stage of a development through to completion.

What Our Clients say:

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Let’s discuss your SAP project!

Claire Holmes
Claire Holmes
Christina T
Christina T
List Bin
List Bin
Had an air test on the house before work starts on re-roofing, installing air barriers, insulation, and MVHR. Excellent flexible service that was exactly what I wanted, with a comprehensive report delivered the next day. It doesn't get much better than this.
Paul Taylor
Paul Taylor
Great service punctual and professional would recommend highly
Jual Ahmed
Jual Ahmed
Excellent service from start to finish. I would highly recommend and will definitely use again. Thank you!