Extensions Question

How to fill the 6 inch gap between a new build extension and the neighbours extension

I am looking at building a 5 m X 3m extension. However the foundations will be 1 meter deep X 600 meters wide with a trench fill depth of 670 cm.

The neighbours has a flat roof and my propsed extension will be pitched.

BCO advise I need to build centrally on the foundations but this would leave a 6 inch gap and inaccessable for maintenance, etc.

How could I block off this gap espectially where the flat roof will meet my extension wall.

I assume I can use single brick to brick off the vertical and in fill the gap with extra insulation but how can I bridge the gaps between the roof and my extension wall.

The altenative advised by BCO is to obtain structural calculations to build 3 cm from the edge of the foundations making it easier to bridge and fill the smaller gap.

I do not want to tie into the neigbours wall as their extension in falling forward so they advised.

Does any body have and advise how I can over come this issue

7 Answers

Best Answer

leave the gap , if there exten is old you could end up with a damp problem, leave the gap as is

Answered 19th Apr 2012

Flintshire Loft and Garage Conversions

Member since 21 Feb 2011

Hi the easiest way would be to purchase a 150mm x 150mm fence post, make sure it's tantalised and get your bricked to build it in between the two extensions there won't be any problems with bco or party wall. If you buy one long enough you can terminate at the top of your extension make sure the bottoms on the footings this will stop cats or rodents getting behind as tantalised timber is rodent proof. This would look better than bricks on edge the top you could use Lead or lead substitute. Most importantly talk to your neibours and let them know what's going on all the way through the build I always make a point of introducing myself to clients neibours before I start work on site this makes for a better enviroment if everybody knows everybody else and issues can be raised and talked about before your neibours go to the bco and this causes a lot of stress, good luck with your build

Answered 18th Jul 2012


Member since 28 Mar 2012

As you said Brick up the gap vertically fill with insulation (be sure to add DPC to prevent damp rising into insulation) I would put a couple of air bricks in, then build a box gutter between your sloped roof to neighbours flat roof and flash into there's to seal properly.

Answered 19th Apr 2012

Personal Buildings

Member since 27 Mar 2012

Leave the gap.

You will need party wall agreement to join to the neighbours' property.

I would place some chicken wire or similar at the top to prevent leaves and debris entering.

Answered 23rd May 2012


Member since 23 May 2012

As stated above, you will need a Party Wall Agreement to cut a flashing into next doors wall. You may also need one if your foundations are going to go to a greater depth than your neighbours foundations, which is likely if your neighbours extension is old as you put it. Your architect/surveyor should be guiding you through this sort of problem.

Answered 6th Jul 2012

Ziggurat Design

Member since 28 Jan 2010

No feedback

The way forward would be to fill the gap with insulation boards near to the top and build a gutter tray with a downpipe on the back garden side and brick the gap at both ends. And the footings have to be at least 1.5 metre deep. Hope it all goes well.

Answered 24th Oct 2017

B A Building Ltd.

Member since 23 Oct 2017

No feedback

Any work to a shared boundary wall would need a Party Wall Agreement.

As stipulated above, the foundations would need to be considered as well. If your foundations are within 3 m of your neighbours foundations, and are to be deeper, you would need a Party Wall Agreement.

Answered 28th Nov 2017

ASJR Architecture

Member since 28 Nov 2017

No feedback

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