Joining cavity wall extension to existing hollow block walls

Roy Rogers · 20-03-2021 9:24pm #1

I'm planning a small rear extension (exempted development). No architect involved, but I'm working with a structural engineer. The existing (detached) house is a hollow block construction, so the builder has suggested a hollow block wall drylined with 102.5mm insulation/plasterboard for the extension too.

Everything I read (here and elsewhere) says to avoid hollow block walls. I'm curious if there's a reasonable argument for using them if the existing walls are the same.

Also, if a cavity wall with a different thickness to the existing walls juts out further to the sides (say 50-100mm or so), does that raise planning issues? Or does it still come under exempted development as there's no internal space added to the side?

Calahonda52 · 20-03-2021 10:40pm

Is the site exposed?

Roy Rogers · 20-03-2021 11:41pm

Not particularly (unless my idea of 'exposed' is off). Last house in a row in a housing estate. House on one side, 8 foot block wall on the other, another house behind. Some small trees (10-12 feet) and a 6 foot fence along the boundaries.

Calahonda52 · 21-03-2021 9:38am

This what I had in mind

Roy Rogers · 21-03-2021 12:09pm

Calahonda52 wrote: »

This what I had in mind

Shows what I know! It's Kildare, so 3 on that chart.

MicktheMan · 21-03-2021 4:18pm

OP, have you (or your builder) considered block on flat with ewi?

Calahonda52 · 21-03-2021 5:22pm

MicktheMan wrote: »

OP, have you (or your builder) considered block on flat with ewi?

Might require a separate rain shield?

Roy Rogers · 21-03-2021 6:25pm

Hadn't considered EWI off the bat for the extension. So it'd be a single leaf wall, block on flat? Didn't even know that was an option to be honest.

I do plan on putting EWI on the existing building, but that won't be for a year or two. Figuring out the best approach now to tie in with that job is one of the things I'm trying to work out.

Calahonda52 · 21-03-2021 6:41pm

Roy Rogers wrote: »

Hadn't considered EWI off the bat for the extension. So it'd be a single leaf wall, block on flat? Didn't even know that was an option to be honest.

I do plan on putting EWI on the existing building, but that won't be for a year or two. Figuring out the best approach now to tie in with that job is one of the things I'm trying to work out.

well the ewi would work well as the block on flat would be in line with the outer face of the hollow block.
You need to reflect on the rain shield for the EWI, depending on your exposure

Roy Rogers · 21-03-2021 7:22pm

Thanks for the tip. Will look into it and raise the question with engineer and builder.

On the planning question: retrofitting EWI would extend a house by 10-20cm each side, yet is generally exempt from planning. Any knowledge on whether the same principle would apply to an extension that sticks out around the same amount?

Joining cavity wall extension to existing hollow block walls

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