Replacing Single Door by 1745mm Opening in Load Bearing Wall

[WoOt]

The previous owner built an addition on to our house, replacing what used to be double doors by a single door. The single door was also moved to the left of where the double door used to be (but is still under the concrete lintel that is already in place and I would guess has always been there).

It makes the room quite dark, so we’re thinking about removing the single door and just leaving a 1745mm opening in the middle (roughly where the old double door was).

There is already a concrete lintel in place where the old door was.

I am now curious whether:

1. I could just remove and brick up the single door and re-open the area under the old lintel.
2. I need planning permission or building regulations to do any of this?
3. I need “option 2” plus have to invest in a builder coming in and replacing the old concrete lintel with and RSJ or adding an RSJ under the old lintel.

I am clueless when it comes to planning permission and building regulations. So if I need either, I wouldn’t mind someone gently pushing me in the right direction for the best steps to follow, or where to get more advice. As far as I can see I definitely won’t need planning permission, but I would need building regs?

Thanks

EDIT: Added an image, which might/may help

 

[Nakajo]

Some photos might help

 

[geraldthehamster]

Are those daleks next to the proposed opening?