terraced house rear extension - boundaries and foundations

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biryani_boy

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hi all,

I have several questions regarding a simple rear extension on a terraced house and I was wondering if anyone here can help with some knowledge. we'd like to know exactly how things work before we start spending money on approvals (restrictive covenants) and drawings.

1. my house is a mid terrace as per drawing below. none of the neighbours has an extension, but there's a shared alleyway between me and neighbour on the right. my property boundary is the thick line drawn, which goes right through the middle of the alleyway once the garden starts.

unnamed.jpg

the neighbour to the left doesn't share a physical wall with me for a portion at the back, their house is offset at the front.

I would like to build a standard single-storey rear extension (around 6m x 3.4m) and I'm interested in how the foundation would work. I know they should be at least 450mm wide, but I'm confused about spacing. what I really want to avoid is having an extension that "shrinks" towards the back, I just hate the concept. we want a nice, long kitchen on one single wall, in one straight line, as if it was built at the same time as the house.

what are the rules around digging the foundations exactly up to the boundary and building a continuation of my existing outer walls? am I tresspassing if I dig the foundation under the neighbour's boundary? if I am, how does a foundation up to the boundary work? do I need an offset footing or do I build 150mm inside my boundary to avoid any issues?

2. obviously, part of building the extension would be to open the wall at the rear. it will disappear entirely. I know an RSJ is needed, but I've heard something about a "goalpost" recently.

am I right in thinking that I can still use the extenal walls (+ let's say another 150mm each side) and a pad stone to install a beam, or do I need two vertical beams on each side of the house as well?

3. any tips on how to calculate the height at the back of the extemsion, where it joins the house? I would like something that looks "natural" and complies with requirements for a pitched roof.

once again, I heard something about a 20 degrees gradient, is that the rule of thumb? existing tiles are concrete and I will likely have to match the materials.

4. while there is little physical connection to the neighbours' walls on either side (at least at the back, where the extension will be), I know that theoretically and legally we share a wall. do I have anything to do with the party wall act in this case?
 
biryani_boy said:
what are the rules around digging the foundations exactly up to the boundary and building a continuation of my existing outer walls? am I tresspassing if I dig the foundation under the neighbour's boundary? if I am, how does a foundation up to the boundary work? do I need an offset footing or do I build 150mm inside my boundary to avoid any issues?
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Foundations may gain some agreed easement. However, you must not overhang the boundary (trespass) with any roof items. This alone means the extension masonry may need to be set back from the boundary.
 
^woody^ said:
Are you drawing the plans yourself?
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sort of. my dad is a civil engineer and he'll draw my plans for free, based on my requirements and real life measurements. all I need to do is decide what's possible within technical/legal/planning constraints and the plans will be done.

noseall said:
Foundations may gain some agreed easement. However, you must not overhang the boundary (trespass) with any roof items. This alone means the extension masonry may need to be set back from the boundary.
Click to expand...

that makes sense. how wide is a typical overhang? would I be able to gain that width back by shrinking the cavity in the side walls? as the kitchen is likely to be wider than 3 meters (extension depth - wall thickness), not having flush walls and losing inches on each side would kind of defeat the purpose.

also, should I opt for an eccentric foundation and build exactly up to the boundary, is there any technical solution for keeping the roof exactly over the extension without any overhang?

thanks both
 
biryani_boy said:
also, should I opt for an eccentric foundation and build exactly up to the boundary, is there any technical solution for keeping the roof exactly over the extension without any overhang?
Click to expand...

Best to discus the proposal with your neighbour, it might give them the idea to do similar, in which case the problem of overhang disappears and the boundary wall can then become a shared wall, with shared foundation and possibly a shared cost.
 
biryani_boy said:
sort of. my dad is a civil engineer and he'll draw my plans for free, based on my requirements
Click to expand...
Oh, he designs bridges and roads?

Well if he knows about eccentric foundations, moment frames and buttressing, you tell him what you want, he designs it. Let him worry about the technicalities - but not so he makes problems for the builders.

You'll need to refer to your council's local planning policy, and the Party Wall Act may apply, unless you design so that it does not, or unless you want it to apply for your own benefit.
 
Do some basic plans with the wall on the boundary and see what happens? You get a couple of goes to resubmit plans if they fail.
Or you could try and organise a pre planning meeting with the council.

I went out 3.3m and managed a 20 degree pitch for concrete tiles and velux windows. A lot depends on how high your top windows are...
 
Presuming the party wall 1 brick thick a cavity wall is going to protrude into the room. Either design the different wall depth into the kitchen, maybe fit a deep worktop and cut to 600 where required, or batten the wall off.
 
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