Raft foundation - close to existing structures

[WisteriaMews]

Does anyone know with a raft foundation how close you can get to existing structures?

 

We have an existing sewer connection on our site with a manhole that drops about 4m and then runs to the boundary and main sewer. The constraints of the site mean we will need to get as close to it as we can. (The quotes for moving the sewer connection have been high.)

 

Separately we have a brick boundary wall that will be the rain screen for the ground floor of our build. To avoid the expense of a demolish and rebuild could the raft get close enough to avoid this?

 

Grateful for anyone's experience!

[mjc55]

i think that you would need to ask whoever designed your foundation.

[WisteriaMews]

We're currently at the planning stage so don't have a foundation designer / structural engineer on board yet. I appreciate these will be the people that need to give a definitive answer but I was wondering if anyone on the forum and done similar and could share their experience.

[Nickfromwales]

9 hours ago, WisteriaMews said:

We're currently at the planning stage so don't have a foundation designer / structural engineer on board yet. I appreciate these will be the people that need to give a definitive answer but I was wondering if anyone on the forum and done similar and could share their experience.

It’s down to geotechnical surveys and input from an SE. Trying to short cut and ask questions like this here are (sorry) a total waste of your time, and nothing typed here will be of any value or use whatsoever as none relate to your site   

 

You can bite the bullet now, and do the geotechnical and grounds conditions surveying, test pits and percolation testing etc done, as you’ll need these to decide on what type of foundation is required regardless, as "feasibility". Also, this will discount the possibility of having to do a piled foundation, in the worst case.

 

With a good engineer anything’s possible, but more info would be needed about where the 4m deep manhole gets its input, from the new house, as you may need to alter this to become a proper backdrop chamber (with such huge inverts).

[saveasteading]

4 hours ago, WisteriaMews said:

if anyone on the forum and done similar and could share their experience.

Yes and no. This would need very detailed consideration and a site survey, just to give this preliminary advice.

After you have had it, may be the time to ask on here for other ideas, which a good SE will be happy to discuss.

 

But do be aware that the term 'raft' is used to mean different things  to different people. Do you mean a heavily reinforced slab that could span over soft spots and support structural walls, or simply a concrete floor?

[Gus Potter]

On 07/03/2026 at 12:33, WisteriaMews said:

We're currently at the planning stage so don't have a foundation designer / structural engineer on board yet. I appreciate these will be the people that need to give a definitive answer but I was wondering if anyone on the forum and done similar and could share their experience.

I design stuff like this. There are many options. Best advice I can give you is to get an SE on board now, even if to provide a watching brief. 

 

If you don't then you introduce significant cost risk and uncertainty.  You know there is a potential issue so the sooner you get that under control the better. 

 

 

 

 

[Nickfromwales]

On 07/03/2026 at 17:32, saveasteading said:

But do be aware that the term 'raft' is used to mean different things  to different people.

Wise words, a lot of people forget.

[lizzieuk1]

On 07/03/2026 at 11:10, WisteriaMews said:

Does anyone know with a raft foundation how close you can get to existing structures?

 

We have an existing sewer connection on our site with a manhole that drops about 4m and then runs to the boundary and main sewer. The constraints of the site mean we will need to get as close to it as we can. (The quotes for moving the sewer connection have been high.)

 

Separately we have a brick boundary wall that will be the rain screen for the ground floor of our build. To avoid the expense of a demolish and rebuild could the raft get close enough to avoid this?

 

Grateful for anyone's experience!

Have you also asked the water board? They may have minimum distance requirements along with max build over distances (I'd try to avoid that though!) 

We are having an insulated raft and have a min of 600mm hardcore beyond the footprint (with integral drainage) all round if that helps. 

[WisteriaMews]

5 minutes ago, lizzieuk1 said:

We are having an insulated raft and have a min of 600mm hardcore beyond the footprint (with integral drainage) all round if that helps. 

Thanks. This is the type of experience from people who have done it I was hoping for 🙂

[Gone West]

On 07/03/2026 at 11:10, WisteriaMews said:

Grateful for anyone's experience!

Sixteen years ago we had an Isoquick 300mm Peripor insulated raft foundation installed. The Structural Engineer specified 200mm compacted type 1 with 50mm thickness of 3mm granite fines on top, type 1 to be extended 700mm beyond outside edge of raft. The raft was 200mm thick with two layers of A393 reinforcing mesh and an additional third layer, 1200mm wide, around edge of two storey part of foundation. All this was for our ground conditions, yours could be very different.

Edited 21 hours ago by Gone West

[Gus Potter]

7 hours ago, WisteriaMews said:

Thanks. This is the type of experience from people who have done it I was hoping for

If you want more targeted feedback then suggest you post your drawings.

 

The key here is to recognise that folk on BH have never seen you project so rather than making them guess just provide as much info as you can. Take title boxes off drawings and identifiable marks if you wish.

[JohnMo]

On 07/03/2026 at 13:27, Nickfromwales said:

Trying to short cut and ask questions like this here are (sorry) a total waste of your time,

Wise words. But seems to the way of things, let's missed out the key steps and try to short cut, then end up spending more time and money to get yourself out of a self dug hole.

[Gus Potter]

30 minutes ago, JohnMo said:

Wise words. But seems to the way of things, let's missed out the key steps and try to short cut, then end up spending more time and money to get yourself out of a self dug hole.

 

On 07/03/2026 at 11:10, WisteriaMews said:

Does anyone know with a raft foundation how close you can get to existing structures?

I kind of think back to when I first joined BH. I took me a while to get my head around how it works. BH is not like some woke Blue Sky place, but I can't really think of anyone who is deliberately horrible. 

 

@WisteriaMews post more info with lots of details as I said before and loads of folk will rally round. 

 

 

 

 

 

 

 

 

 

[Nickfromwales]

11 hours ago, JohnMo said:

Wise words. But seems to the way of things, let's missed out the key steps and try to short cut, then end up spending more time and money to get yourself out of a self dug hole.

I’m up against this constantly. My attempts to steer folk away from expensive and time consuming misadventures is often rebuffed at first, then it’s an 11th hr dash to douse the flames with dampened £50 notes.