This one is genuinely unclear. I got my BER done after both the attic conversion and solar panels, so I can't isolate the conversion's impact. Here's what I do know.
My house was rated A2 before the conversion. After the attic conversion and then solar panels a few months later, I got the BER reassessed and came out at A1. But the assessor seemed to find reaching A1 a challenge - it wasn't a comfortable margin. That makes me think the conversion alone could have brought the rating down, and the solar panels pulled it back up and beyond. That's speculation on my part, but it's an honest read of the situation.
A2
Before conversion
+ Solar
Added a few months later
A1
After both
How BER works for conversions
A BER (Building Energy Rating) measures the overall energy efficiency of your home. It takes into account insulation, heating systems, windows, lighting, ventilation, and the total floor area. The rating runs from A1 (most efficient) down to G (least efficient).
Here's the thing most people don't realise: an attic conversion changes the equation in both directions. You're adding insulation (good), but you're also adding habitable floor area that needs to be heated (potentially bad). The net effect depends on the specifics of your conversion - insulation type, window spec, number of radiators, and how well everything is fitted.
It's genuinely hard to predict the outcome. The conversion could improve your rating, maintain it, or decrease it. It depends on too many variables for a simple answer.
Factors that help your BER
Good insulation with a low U-value
The better the insulation, the less heat escapes through the roof. Spray foam typically achieves a better U-value than rigid board, though there are trade-offs — see our insulation guide comparing spray foam and Kingspan for the full picture.
High-spec Velux windows
Modern Velux windows have significantly better thermal performance than older models. A lower U-value means less heat loss through the glass. Most current Velux windows are double or triple glazed and perform well — our Velux windows and blinds guide covers what to look for.
LED lighting throughout
LED lighting uses a fraction of the energy of older bulb types. If your conversion is fully LED (which it should be), this helps the overall calculation.
Solar panels (if you’re also adding them)
Solar panels significantly boost your BER. If you’re planning both a conversion and solar, get the BER assessed after both are complete — the solar will more than offset any conversion impact.
Factors that could hurt your BER
Adding habitable floor area
More floor area means more space to heat. If the insulation in the new space isn’t good enough to compensate, the overall rating drops. This is the main risk.
Additional radiators
Each radiator added to the attic increases the heating demand the BER assessor calculates. More radiators = more energy required to maintain temperature. See our radiator and heating guide for what's typical.
Poor insulation or gaps
Badly fitted insulation, gaps around windows, or thermal bridges where the new structure meets the old can all undermine what should be a well-insulated space.
Lower-spec windows
If the builder uses cheaper roof windows with a higher U-value, more heat escapes through the glass. Always ask what spec windows are included in the quote.
What to discuss with your builder
The key takeaway here isn't that you need to become a BER expert. It's that you should be aware of the potential impact and have the conversation with your builder before the work starts.
A builder doing regular attic conversions should have a sense of how their work typically affects the BER. They may have options to maximise your chances of maintaining or improving your rating - better insulation, higher-spec windows, or construction details that reduce thermal bridging. Ask them directly:
"In your experience, how does an attic conversion affect the BER?"
A good builder will have done enough conversions to have a feel for this. If they’ve never thought about it, that tells you something.
"What U-value does your insulation achieve?"
The lower the U-value, the better the insulation performs. This is measurable and the builder should know the answer.
"What U-value are the Velux windows you’re specifying?"
Modern Velux windows vary in thermal performance. Higher-spec models cost more but lose less heat.
"Is there anything you can do to maximise the BER outcome?"
There may be relatively simple changes — thicker insulation, better window spec, attention to thermal bridges — that make a real difference to the assessment.
The bottom line
I can't tell you with certainty whether an attic conversion will help or hurt your BER. My own experience - A2 to A1 - included solar panels, so the conversion's contribution is impossible to separate. I'd be open to the possibility that the conversion alone could have decreased the rating, and the solar pulled it back.
What I can say is: make sure the basics are right. Modern Velux windows, properly fitted insulation with a good U-value, and LED lighting throughout. Have the conversation with your builder about BER impact before the work starts. And if you're planning solar panels anyway, get the BER assessed after everything is done - the combined result is what matters.
If you're also adding solar
Solar panels will almost certainly more than compensate for any BER impact from the conversion. If both are on your radar, don't stress about the conversion's individual effect. Get everything done, then assess.
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