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, and A1 after the attic conversion and solar panels together — both of those under the 15-band scale that Ireland used until 24 May 2026. Neither label exists any more. 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 (old scale)
+ Solar
Added a few months later
A1
After both (old scale)
Both ratings are on the 15-band scale Ireland used until 24 May 2026. Neither band exists any more.
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 A0 (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.
The scale changed in May 2026
On 24 May 2026 the BER scale was simplified. The sub-categories — A1, A2, A3, B1, B2, B3 and the rest — were removed, and the number of levels shown on the certificate went from 15 to eight: A0, A, B, C, D, E, F and G. The change came in under S.I. No. 168 of 2026, the regulations transposing the recast EU Energy Performance of Buildings Directive, and it brings Ireland into line with the rest of the EU.
A0 is a new top band, for zero-emission homes. The regulations define a zero-emission building as one with a very high energy performance, needing zero or a very low amount of energy, producing zero on-site carbon emissions from fossil fuels, and producing zero or a very low amount of operational greenhouse gas emissions. A house heated by a gas or oil boiler produces on-site emissions from fossil fuels, so it is outside that definition.
A BER certificate is valid for ten years from its date of issue, but only while the building it describes has not materially changed. Regulation 22 of the 2012 Energy Performance of Buildings Regulations renders a certificate invalid if there is “any material change in the building to which it relates which could affect its energy performance”, and it gives three examples: any significant deterioration in the fabric of the building, any extension of the building, or any change in the heating system or in the type of fuel it uses. That list is introduced with “including”, so it is illustrative rather than exhaustive.
Match that against what your job actually involved, rather than against the word “conversion”. We are not going to tell you which side of the line you are on: whether your job meets that test turns on facts about your building that we cannot see, and a BER assessor is who to put the question to. If a sale or a letting depends on the certificate, that is the question to put to them. A currently valid BER still satisfies the obligations that come with selling or renting.
There is a catch for anyone judging a conversion by the letter. With eight bands instead of fifteen, each band is wider, so a conversion has to move the underlying figure further before the letter moves at all. A rating that came back unchanged is not evidence that nothing changed. A certificate issued since 24 May 2026 carries more than the letter: alongside the band it reports energy use, renewable energy contribution, and greenhouse gas emissions and global warming potential, and it carries a QR code linking to information about the home's rating and its upgrade options. Ask your assessor for the figures, not just the band.
Source: S.I. No. 168/2026 - European Union (Energy Performance of Buildings) Regulations 2026
Source: S.I. No. 243/2012 - European Union (Energy Performance of Buildings) Regulations 2012, Regulation 22
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 on the old scale — 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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