To be concerned about the new extension?

[NonTraditionalFeelings]

I've just been through a very long and painful experience of getting a house extension done. Will never do it again! It's nearing its end but I'm really concerned about the temperature in the extension. Without the heating on this weekend it was about 19/18 degrees in the main house, it dropped to 14 degrees in the extension. (It's empty at the moment as it's not 100% finished, skirting is going on etc).

My builder thinks I'm being unreasonable as in his words all of the insulation is up to building regs. It's a flat roof garage conversion with patio doors and a side extension to connect the two.

Am I being unreasonable? It wasn't that cold on Sunday and I wouldn't have wanted to be in there without the heating on. It's going to be my new lounge so now I'm having a panic attack that it's not been done correctly.

He says it's still drying out which will make it colder, ok fair enough but 4 degrees colder? It doesn't warm up with the heating on but gets cold very quickly.

Has anyone experienced anything similar? AIBU to question him on this?

Where do you live that it was that warm without the heating on? No advice though sorry but if it’s got more exposed walls it will likely be colder than the rest of the house

From your description I'm imagining it was a free standing garage initially that you've joined onto your house?

Presumably the garage was single brick and now has a second breeze block wall with insulation between and the floor raised.

Patio doors are cold but once you've got your flooring down and curtains up it'll be much warmer.

My extension is always colder - it's because the rooms in the original part of the house now effectively only have 1 external wall (neighbour one side, extension on the other) making the old part now a few degrees warmer.

The rooms in the extension have 2 external walls meeting at a corner, one of which is the gable end - lots more scope to lose heat so a couple of degrees cooler!

My garage conversion is always freezing in the winter. It's not ideal but it was way cheaper than knocking it down and starting an extension from scratch. Big bifold doors don't help. We don't spend much time in there in the winter but in the summer it's lovely, overlooking open meadows and it's my favourite room.

We had a protruding extension with really naff plastic roof (I've mentioned it on threads here before cos it sucked) which actually melted and sagged in the heatwave of 2018. So we had the roof replaced this year with a proper thermally insulated roof.

It's cold outside now but the temperature in our extension is normal, even without heating in that part. It's open to the dining room (with rad at the far end) and kitchen (rad ditto) and it feels OK. I would expect it to be a few degrees colder, since it has three outside walls and no heating of its own.

Our extension has sliding patio doors on one side, big picture window at the end and a row of obscure glass windows down the length and it's still managing to feel reasonable, even after dark (17 degrees now by the picture window), so I wonder if the problem is your roof?

I'm concerned that you say it doesn't warm up with the heating on, surely it should?