You’ve seen it before: two homes on the same street, same layout, same square footage—even the same builder. Yet somehow, one has an EPC rating of C, the other only scrapes a D. What gives?
In 2025, as Energy Performance Certificates play a growing role in property value, rentability, and compliance, it’s more important than ever to understand why EPC ratings vary—even between “identical” properties.
🏠 On the Surface: Identical Specs, Identical Design
You might assume that two properties with:
The same construction materials
The same insulation package
The same boiler model
The same windows and orientation
…should logically receive the same EPC rating.
But EPCs are not just about blueprints—they’re about real-world condition, user behaviour, and hidden performance factors.
🔍 So Why the Discrepancy?
Let’s break it down.
1. Installation Quality Varies—Even in New Builds
Just because two homes were built with the same spec sheet doesn’t mean they were finished to the same standard. For example:
One property may have air gaps in insulation
Another might have poor sealing around windows
Ducting, pipe lagging, and radiator balancing may differ
These micro-differences can have macro impacts on thermal efficiency.
2. Heating System Settings & Controls
EPC assessors record:
Thermostat presence and positioning
Programmers or smart controls
Efficiency ratings of installed systems
If one homeowner upgraded to smart zoning or better boiler controls, that alone could push them up a rating band—even with the same boiler unit.
3. Loft & Cavity Insulation Degrades Over Time
In older homes, insulation can settle, compress, or become damp, which lowers its performance. If one homeowner topped up their insulation recently, while the other didn’t, this difference is captured in the EPC.
4. Glazing and Frame Types May Appear the Same—But Aren’t
Two windows may look identical, but EPC software assesses:
Frame material (wood, uPVC, aluminium)
Glazing type (double vs triple, gas-filled or not)
Gap width between panes
Subtle differences here can impact U-values and EPC banding.
5. Ventilation Matters More Than You Think
Mechanical ventilation systems (with or without heat recovery) affect EPC outcomes. Even bathroom extractors or trickle vents are recorded.
Poorly ventilated homes may get penalised for potential condensation and heat loss, even if the thermal envelope is good.
6. Assessor Subjectivity + Software Tolerances
EPC ratings are generated using RdSAP, a standardised software. But:
Assessors must input dozens of assumptions
Missing evidence = default low rating
Two different assessors may interpret unclear data differently
So even identical homes assessed by different people—or at different times—can end up with different ratings.
7. Improvements Not Registered
If one homeowner installed:
A new combi boiler
Solar PV
Extra insulation
Low-energy lighting
…but didn’t provide evidence during the EPC assessment, those improvements won’t count in the score.
📊 Real-World Case
Two semi-detached homes in North London:
House A: EPC D (68)
House B: EPC C (72)
After investigation, House B had:
Smart thermostat with zoning
Topped-up loft insulation
Trickle vents in windows
Documented boiler upgrade
Slightly better airtightness from professional draught-proofing
The result? A full band jump, despite nearly identical appearances.
💡 How to Maximise Your Own EPC
Want to avoid being the “underperforming twin”? Here’s what to do:
Document everything: Keep receipts and datasheets for insulation, glazing, boiler upgrades, etc.
Book a thorough assessor: Some providers rush through—choose one who takes time.
Install low-energy lighting throughout
Add or upgrade heating controls
Seal draughts and consider MVHR or passive ventilation
📞 Need Expert EPC Advice?
At EPCRate London, we don’t just hand you a number—we help you understand and improve it.
Final Word
Two homes may look identical—but EPCs reveal the invisible differences in thermal efficiency, energy systems, and build quality. If you’re serious about energy performance, don’t just aim to match your neighbour—aim to outperform.
