Energy surveys and installing big windows/bifold doors - does anyone know anything about this please?

[ThatsNotMyCow]

We're getting plans drawn up for an extension and the designer wants to put in much smaller windows and bifold doors than we would like in order to avoid an obligation to do some sort of energy survey. I can't find anything about this from googling - I assume I'm using the wrong terms - does anyone have any experience of getting bifold doors or similar installed and having to have an energy survey because of the area of the glass? Is it very expensive or otherwise a huge hassle?

My feeling is that it's better to pay for an extra survey than have a dark and dingy extension (I can't think that an extra energy survey is going to be that expensive in the context of a whole building project) but as our designer seems so keen to avoid it I wonder if this is actually a big deal?

I'd be very grateful if anyone could shed any light on this! I'd like some second opinions before overruling the designer in case I'm making a daft decision.

What you are looking for is a SAP test. If the area of glazing is over 25% of the extension floor area you require one of these to show compliance with part L2 of the building regs. You may also need to up your insulation levels to compensate for the thermal loss through glazing in order to pass the test.
I don't recommend this company I just got their details through google but it explains it in more detail.
www.energy-test.co.uk/content/10/sap-calculations-for-heavily-glazed-extensions

Thank you! Looks like we might have to improve some insulation elsewhere in order to have lots of glass - it's something we've been meaning to do anyway. Although I wonder, given how common extensions with giant bifolds are atm, whether you can get glazing that counts as highly insulating for the calculation, or have a superinsulated roof and walls or something!

Also look at the type of glass you are using.
The U value measure a rate of heat leaving the building through different surfaces. You want lower the better.
Some glass has U value of around 1 - which is really good.
Google celsius glass.

I've just had full extension plans passed by Building Regs which include 5.5m of glass doors and 2 rooflights. The plans had to include a SAP calculation document showing the 'to be' extension to be more efficient energy wise. The first architect i approached quoted £600 just for the SAP calculations but the second one had it incorporated into price so was nowhere near this amount. To be honest, I think your designer should be listening to what you want rather than taking the easiest route.

Thanks, I've done some googling on U values etc. I do hope the designer knows how to design an extension that will pass the SAP calculation, it is not inspiring confidence that he wants to avoid it altogether rather than design something that looks better and complies.