When extending should the line of the new cavity/block be flush with the existing cavity/block?

[paul.uk.25]

I'm looking to extend the kitchen to the back of my house by removing the rear wall and extending outwards by a further 2m continuing the line of the existing walls on either side.

Now the existing external wall is 265mm wide and I think current standards mean my new external wall (which I would like to run flush externally to the existing) should be closer to 330mm wide with a much larger cavity.

I'm just wondering what would be considered best practice here when setting out the new wall.

- Should the new wall buildup match the 265mm of the old wall buildup so that the internal blockwork and cavity line through. Then line both internally with insulation backed plasterboard to bring them up to current standards.

Or

- Should the new wall be built to current standards of 330mm with a larger cavity so that the new blockwork does not line through with the existing blockwork. The old wall buildup would then be internally lined with insulation backed plasterboard to bring it to current standards and so they both line through flush with one another.

I'm not sure if it makes any difference but the eaves on one side is at first floor level. So that when the kitchen is extended the eaves and and roof on that side will extend out flush with the existing as well.

Thanks in advance!

 

[tomfe]

I would make the cavity bigger, you don't gain anything from having the internal leaf lineing up with the original but you will save money on the price of thermal pb.
Iam not sure what you mean about the roof, are you doing a 2 story extension?

 

[^woody^]

A new extension wall should be designed ( and built) so the the internal plaster finishes flush with the existing, the wall meets the required u values, and the eaves and soffits line through with the existing.

 

[noseall]

A new extension wall should be designed ( and built) so the the internal plaster finishes flush with the existing, the wall meets the required u values, and the eaves and soffits line through with the existing.

Agree Woods, but it is not always possible to line up the masonry, in particular when adhering to latest standards. It is more important to get the outside to work and line up and then add cosmetic thermal stuff to the inside in order to gain continuity of finishes.

 

[paul.uk.25]

Thank you for the responses.

So it looks like it would be best to construct the wall to latest standards with a larger cavity and ensure that all plasterboard is lined through afterwards.

With regards the roof. This is a single storey extension. The height of one side of the house is the same height as the extension so the new roof will just be a continuation of the existing. I was concerned that not lining up the internal wall might complicate the ability to match up the existing and proposed roof line and keep everything flush.

Thanks again!

 

[^woody^]

So it looks like it would be best to construct the wall to latest standards with a larger cavity

No. The latest standards do not mean a larger cavity.

If a flush plaster finish is the main criteria, then the design of the wall should be to meet that, and there are several options for wall design and thickness.