Husband and I debating how to extend

[Daisypod]

I’ve attached a diagram but it’s not great! Sorry if this ends up being long and confusing!
One of our kids now has their bedroom in the extension as no one wanted to share so we built a wall between that bit and the dining room, just a basic frame and plaster board one.
We are now going to have an extension for a new big kitchen to the back of the house. Originally it was going to be open plan dining room kitchen with the extension room and utility being part of it and one big space. DS would have the bedroom in the old dining room, no windows unfortunately but as he works nights he doesn’t like the light anyway. Now Dh is suggesting that we make the extension bigger and keep the extension as another separate room to be a study. But this would mean two rooms downstairs with no windows. The old kitchen would be a utility room. There is a possibility of putting velux windows in the old extension room though.
i think that’s too many rooms with no windows. I’m not keen on having any but needs must at the moment and when DS moves out we can take down that wall and maybe put in glass doors or something.
I think making the old kitchen into the study and forgoing a utility would be better.
Weve never done anything on this scale before and it’s rather daunting especially as I’m autistic and struggle with change and having people I don’t know in the house!
So can any mumsnetters share their wisdom on the situation?

I think your original plan with DS having the dining room as a bedroom is a good one, especially if he's not going to be there very long. Keep the other rooms as airy and light as possible.

Building Regulations for Windows in Bedrooms | Checkatrade
In the UK, building regulations require that every room used for sleeping must have a window for ventilation and natural light. This is to ensure that occupants have a safe exit in case of an emergency. The window must be able to open wide enough to allow a person to escape in case of a fire, with a clear openable area that’s at least 0.33 square metres. It is illegal to let or stay in a room without a window, as it does not meet the legal definition of a bedroom.

Is there scope for creating a light well?

Can the living room not be your sons bedroom in the short term, with the dining room/extension being a living room with sliding glass doors into the new extension? When your son moves out you can then always move the living room back to the front.

This is the best idea - no one wants a room without windows tbh

Great idea. It's not safe or comfortable to sleep in a windowless room .