How thin can an external wall be?

[Unforgiven]

We have an existing enclosed porch that is glass/aluminium and is not used as we have another front door to the house.
We have applied for and been granted an LDC to rebuild in brick and convert it into a toilet/cloakroom.
The problem we have is it is only 1.5x1.5m and our builder wants the walls to be 350mm thick which isn't really leaving much room internally.
Am I correct in thinking that the U value only needs to be 30 as it is an existing element in an existing building?
If so what is the thinnest wall I can get away with (heating will be electric underfloor, separate to the house)?
P.S the external wall must be brick.

Many thanks

 

[securespark]

You could insulate externally, then put brick slips on to cover it?

 

[noseall]

Am I correct in thinking that the U value only needs to be 30 as it is an existing element in an existing building?

If you are converting to habitable space, no. You will inevitably fall under the the '25% thermal element' rule in any case.

0.18 W/m2K​

There are SAP calc's or clever work-arounds that may allow you to beef up the insulation elsewhere (other than the external walls) affording you more space inside. All will likely be disruptive or come at a cost.

 

[cdbe]

I don't know what the layout is, but if it's built around an external wall, you could lintel over, remove both skins and replace with a thin stud wall.

 

[freddiemercurystwin]

To achieve 0.18

If traditional cavity wall about 315mm (no service zone)

If timber frame for the internal skin about 325mm (no service zone)

If timber frame (90mm) + brick slips on about 230mm (no service zone)

These are just after some number crunching on the Celotex U Value Calculator, there maybe some enhancements that can be made to get a little thinner than these ie changing materials, going narrower on the studs than the calculator allows etc.

Then as mentioned there are SAP calcs but probably won't shave much of these figures.