Removing French doors leading to extension

I've purchased a house that has a (4.2mx3.2m) extension at the back. Access is via upvc double French doors from the kitchen. The doors were external doors, although are now internal use.

The extension seems well insulated (it's warmer than the rest of the house), and has access to the side alleyway and back garden via more upvc doors.

I have a building control "certificate of completion" for the extension, issued in 2016.

Anyhow, I'd like to remove these French doors so the extension is open to the rest of the house (and it's looks odd having external doors in the house).

I mentioned this to a friend, who said that the external doors being left in may have been a deliberate choice, something to do with building control and the thermal properties of the extension.

Something about having the doors means its separate to the rest of the house and wouldn't have to be tested for insulation properties etc.

Is he talking rubbish? Will there be a building control issue when I come to sell then house if the doors are gone? The building control certificate makes no mention of these, just that the work as been inspected and they're not aware of any contravention to building regs.

Any ideas or advice for me?

Your friend is not talking rubbish.

I don't know much about the subject so please bear with me.

So an extension that is separated from the main house by external doors has different regs that one that doesn't.

Do you get a different building control certificate for one without the external doors separating it from the house?

Am I allowed to remove the doors? Apart from potentially spending more on heating, what are the potential consequences?

I'm guessing I'd need to add them back when selling the house?

Can anyone provide more information or a link about needing to keep the external doors? The building control planning portal talk about removing French doors in extensions without a caveat for thermal efficiency:

"Once an extension has been made weather-proof an opening is normally made through the existing external walls.

This can be achieved by removing any existing French doors, patio or window openings..."

https://www.planningportal.co.uk/info/200130/common_projects/17/extensions/9

Confused...

It's normally a scenario associated with conservatories that would never pass the thermal aspects of the regs, and are typically structurally inadequate due to being built on shallow foundations etc.

As you've not included photos of your build it's difficult for anyone to speculate on whether you have a 'lightweight' extension that's escaped some aspect of the regs for some reason, but it's unusual to have retained the internal separating doors given the modern obsession with knock-throughs and open-plan space, so you've got to wonder...

^^ that covers it. Plus possible separation of the kitchen.

You'll need to check with the council for the specifics of the approval.

I spoke with building control, and emailed a copy of the certificate of completion and photos.

The chap who did the inspection for the certificate has responded that he sees no reason why I cannot remove them and I don't need approval to remove them.

So sounds like it's all ok and I can get rid of them.