Adding an extension or conservatory can transform a property’s space, functionality, and market value. However, these additions also influence your property’s Energy Performance Certificate (EPC) rating. EPC assessors must evaluate how these structures impact the overall energy efficiency of the building, which can be complex depending on construction methods, glazing types, and heating strategies.
In this guide, we explain how EPC assessments account for property extensions and conservatories — and how you can optimise your EPC rating when planning or upgrading these features.
For expert EPC assessments that accurately evaluate extensions and conservatories, visit our Services page.
1. Key Factors EPC Assessors Consider for Extensions
When evaluating extensions, EPC assessors focus on:
Construction type (e.g., cavity wall, timber frame, solid wall).
Level of insulation in walls, floors, and roofs.
Window and door types (single, double, triple glazing).
Heating system connectivity — is the extension heated by the main system?
Air tightness and draught-proofing.
Extensions with modern insulation and efficient glazing generally improve a property’s thermal efficiency and EPC score. However, if an extension lacks proper insulation or is poorly integrated with the main heating system, it can lower the overall EPC rating.
2. How Conservatories Affect EPC Scores
Conservatories, especially older ones, can negatively impact EPC outcomes due to:
High heat loss through large glazed areas.
Overheating in summer, increasing cooling demands.
Draughts if not properly sealed to the main structure.
EPC assessors classify conservatories as:
| Conservatory Type | EPC Assessment Impact |
|---|---|
| Unheated, thermally separated (external doors in place) | Often excluded from EPC calculations. Minimal impact. |
| Heated, thermally integrated (external doors removed) | Treated as part of the main building. High heat loss penalties if poorly insulated. |
For properties with integrated conservatories, upgrades like solar control glazing, underfloor insulation, and smart heating controls can mitigate EPC penalties.
3. Assessing New Extensions: SAP Calculations vs RdSAP
New builds and significant extensions (>25% of existing floor area) require a SAP (Standard Assessment Procedure) calculation during construction.
Smaller extensions or post-build evaluations use RdSAP (Reduced Data SAP) for EPCs, which rely on existing building data and assumptions unless documented upgrades are provided.
Ensure your builder provides:
U-values of walls, roofs, and floors.
Glazing specifications.
Heating integration details.
Air tightness test results (if available).
Provide this documentation to your EPC assessor to avoid default assumptions that may lower your score. You can schedule an assessment with EPCrate via our Booking page.
4. EPC Challenges in Older Extensions and Conservatories
| Challenge | EPC Impact |
|---|---|
| Single-glazed conservatory windows | High heat loss; significant EPC penalties. |
| Poorly insulated flat-roof extensions | Increased heat loss affecting overall efficiency. |
| Lack of integrated heating controls | Overheating or under-heating, reducing efficiency. |
| Draughty join between old and new structures | Air leakage penalties in EPC calculation. |
5. Tips to Optimise EPC Scores with Extensions and Conservatories
Ensure Modern Insulation Levels: Use cavity wall insulation, insulated floor slabs, and roof insulation that meets current Building Regulations.
Upgrade Glazing: Opt for low-E double or triple glazing with thermal spacers.
Maintain Thermal Separation for Conservatories: Keeping external-grade doors between the conservatory and main house prevents energy penalties.
Integrate Smart Heating Controls: For heated extensions, use programmable thermostats or TRVs to manage temperatures efficiently.
Draught-Proof Junctions: Seal all joins between old and new structures to prevent air leakage.
6. Case Study: Rear Extension with Integrated Conservatory
A 1930s semi-detached home underwent a rear extension with an integrated conservatory. Initially, the EPC score dropped due to:
Single-glazed conservatory.
Uninsulated flat roof.
Lack of zonal heating controls.
Post-upgrade:
Installed A-rated double-glazed units.
Retrofitted 150mm flat roof insulation.
Added TRVs on radiators in the extension.
Sealed air leakage points at wall junctions.
EPC rating improved from D to C, making the property MEES compliant and enhancing market appeal.
For a breakdown of similar improvement strategies, explore our Pricing page.
7. Documenting Upgrades for Accurate EPC Assessment
To ensure EPC assessors accurately reflect your extension and conservatory improvements:
Provide U-value certificates.
Offer photos of insulation installation.
Share heating system integration details.
Highlight zoning controls and smart thermostats.
At EPCrate, we ensure no efficiency upgrade goes unrecognised in your EPC report. Learn more about our detailed approach on our About Us page.
Conclusion
Extensions and conservatories can enhance property value but need careful consideration in terms of energy efficiency to avoid EPC penalties. By investing in insulation, glazing upgrades, smart heating controls, and effective air sealing, property owners can ensure these additions contribute positively to EPC ratings.
At EPCrate, we specialise in EPC assessments for properties with complex layouts, extensions, and conservatories. Book your assessment via our Booking page or contact us through our Contact page for expert advice.
