Enclosed side passageway - single skin?

[PaulHig]

Hi. First time posting but could really do with some info.
Down one side of my house I have a pathway 1.6m wide at the front and 1m wide at the back. It's 9m long. Gate at the front and open in to my garden at the back. Tarmac floor. About 6m of the 9m there is a boundary fence to my neighbour and 3m it is my neighbours detached garage (no fence. I see wall).
I have an existing side about half way along.
I want to make this an inside space. For storage, maybe the washing machine.
As width is not on my side (especially at the back) I was thinking a proper cavity wall brick and block front and back but single skin with columns down the side. I even toyed with no wall at all where their garage is but think I will single skin alongside this.

I'll likely do a proper insulation roof and insulated floor but it's the width that I really need to save.

Does anyone see any issues with this proposal?
Either in quality (I.e cold, damp) or with things like selling and mortgaging down the line?
Any other options that also save width?

Also not sure if I need building regs.

I'm keeping my existing UPVC side door so it's pretty much an outbuilding in my mind and not an extension.

Can anyone give me some guidance please!

First time poster but I'm really stuck! Can't find any solid advice online and I have done planning applications for extensions etc before.

Thank you!

Paul

 

[blup]

A side extension is deemed pd but there are obviously limits, just search the planning portal. There may be (fire related) building regs implications as you will be on or near the boundary.

 

[PaulHig]

Thanks blup. I wasn't really sure (I'm still not) what differentiated an extension from an outbuilding or a conservatory for example.
I had heard that because I'm keeping the existing external walls and doors that perhaps this would not be classed as an extension