Big Windows or Bifold Doors? Why You Might Need SAP Calculations for Your Extension
If you’re planning an extension with large windows, rooflights or bifold doors, there’s a good chance you’ll need SAP calculations to meet Building Regulations. So, what are Design Flexibility SAP calculations, and why do they matter for your project?
At Plan It UK, we design home extensions across London, Kent, and Essex. As part of our mission to guide clients through every stage of the process, this guide will explain what these calculations are, when they’re required, and how they can help you get more from your design.
When Are SAP Calculations Required?
SAP stands for Standard Assessment Procedure, the UK Government’s method for measuring the energy efficiency of homes.
However, here’s the important part for homeowners planning an extension:
SAP calculations are only required when the amount of glazing in your extension exceeds 25% of the extension’s floor area (after subtracting any glazing that’s being removed as part of the build).
This rule comes from Building Regulations Part L, which sets out energy efficiency standards for extensions.
If your design includes large sliding doors, bifolds, roof lanterns, or lots of windows, it may go over that 25% threshold — in which case, a SAP calculation will be required to show that the extension still meets energy performance standards.
What Are Design Flexibility SAP Calculations?
Rather than waiting until the final design is complete, we carry out Design Flexibility SAP calculations early in the design process to test different options and ensure your vision stays on track.
These are not final energy certificates — they’re early-stage assessments that explore how flexible your design can be in terms of glazing size, insulation levels, and layout, while still complying with Building Control.
It’s like running a feasibility test: how big can those bifolds be? Can we keep the vaulted ceiling? What type of roof glazing is possible?
Why They Matter for Building Control
If your extension has more than 25% glazing, Building Control will require SAP calculations to prove the design meets energy efficiency requirements. Without this evidence, your application may be delayed or rejected.
By carrying out Design Flexibility SAP calculations early, we:
Identify potential issues before they arise
Save time in the approval process
Allow for design changes without compromising compliance
Avoid surprises during construction
The Benefits of Design Flexibility SAP Calculations
Here’s how they support your home extension:
Unlock Larger Glazing Designs Want more light or panoramic doors? SAP calculations help justify bigger openings.
Stay Compliant – Make sure Building Control has the evidence they need.
Get Creative With Confidence – Explore features like vaulted ceilings or open-plan spaces without guesswork.
Avoid Costly Redesigns – Fixing energy compliance late in the build can be expensive and disruptive.
Optimise Energy Performance – A well-designed extension is more comfortable and cheaper to run.
How Plan It UK Supports You
At Plan It UK, we carry out early-stage SAP design assessments whenever large amounts of glazing are proposed. We work with trusted energy assessors to ensure your design is creative, compliant, and future-ready.
We take care of the technical checks so you can focus on bringing your vision to life.
Planning a home extension with large windows or lots of glass?
Let’s make sure it performs as well as it looks!. Contact us or book in your free consultation to get a quote for your project.
