SAP Calculations for Extensions

Consider a single-story rear extension measuring 6m × 4m, giving you 24 square meters of floor area. Under current regulations, you could install up to 6 square meters of glazing (windows, doors, rooflights) without requiring heat loss calculations.

Exceed this amount, and you’ll need to demonstrate compliance through the SAP process.

This calculation becomes particularly relevant for extensions featuring:
- Large bi-fold door systems
- Floor-to-ceiling windows
- Substantial rooflights or skylights
- Multiple glazed elements combined

SAP calculations for extensions compare two theoretical scenarios: the notional extension and your proposed extension design. Understanding this comparison is crucial for achieving compliance while maintaining design flexibility.

Notional Extension Baseline
The notional extension represents a compliant baseline—essentially your existing house plus a theoretical extension that meets minimum standards with exactly 25% glazing. This creates the benchmark against which your actual design is measured.

The notional report establishes target emission rates (TER) and target fabric energy efficiency (TFEE) values that your proposed extension must not exceed. Think of this as setting the energy performance bar that your design needs to clear.

Actual Dwelling Assessment
Your proposed extension, combined with the existing structure, forms the “actual dwelling” for SAP assessment. This calculation determines the dwelling emission rate (DER) and dwelling fabric energy efficiency (DFEE) based on your specific design choices.

To achieve compliance, your actual dwelling’s emissions must not exceed those of the notional extension. This comparison-based approach provides flexibility—you can have more glazing than the 25% baseline if you compensate through other energy efficiency improvements.

SAP calculations for extensions represent far more than regulatory compliance—they’re your pathway to achieving ambitious, highly glazed designs while maintaining energy efficiency standards. Whether you’re planning a modest single-story addition or an extensive multi-room extension, understanding SAP requirements early in the design process opens up possibilities that many homeowners don’t realize exist.

The key to success lies in early professional engagement, strategic thinking about whole-dwelling performance, and willingness to invest in high-quality building elements that deliver both compliance and long-term value. With the right approach, your extension can feature the spectacular glazing you envision while meeting—or exceeding—all regulatory requirements.

Don’t let glazing restrictions limit your design ambitions. Professional SAP calculations can transform regulatory challenges into opportunities for creating truly exceptional living spaces that comply with current regulations and deliver outstanding energy performance for years to come.