Raft foundations

Little_Missy

Hi all,

I'm going to build a small single storey extension (approx. 8' x 13') at the rear of my semi-detached house and am trying to avoid the works falling under the Party Wall etc. Act 1996.

I don't want a big gap between the extension and the boundary, which would be unavoidable with standard foundations unless they project under my neighbours land, so I've been looking at raft foundations. I plan to construct the raft 1" from the line of junction in order to avoid the PWA and, from what I've found on the internet, I would be able to build the outer wall close to the edge of the raft. The ground is not too firm (soft sticky clay), so, again, from what I've read, raft foundations would appear to be the right choice?

Just wondering if anybody has any experience of constructing raft foundations and building off them?......If anybody can share any helpful info re. constructing the raft and also minimum distance, if any, that the outer wall should be built from the very outer edge of the raft that'd be great?

Also, I plan to build a double storey extension next to the single storey one later in the year, but I need the single storey one built now. Would I be better constructing the raft foundation across the entire area now (including the area for the double storey extension) or constructing the other foundations at the time of building the other extension?......From what I've read, I'm not too sure that raft foundations are suitable for double storey extensions?

Thanks in advance!

Little_Missy

Hellooo?.....Anybody?

Well.....this is the first time that I haven't received even a single response to a question

If anybody can be of assistance with this it'd be very much appreciated!

Thanks.

teneighty

OK I'll have a go but bear in mind that this is a technical subject so I will massively over-simplify my answer.


Yes you can have a raft foundation for a small domestic extension, single or two storey but it would have to be designed by a structural engineer. The engineer would be able to advise how close you could build to the edge of the toe but I can see no reason why it could not be right on the edge.


Personally I would only use a raft for a domestic extension as a last resort. As the name suggests a raft is designed to "float" on the sub-soil so with your sticky soft clay sub-soil you will get considerable seasonal vertical movement in the extension. In itself that is not a problem but the joint between the extension and old house on traditional foundation will have to accommodate this movement. Also things like drains, water pipes etc. need to accommodate this movement to prevent fracturing the pipes.


Unless you have an on-going dispute with your neighbour the Party Wall Act does not need to be a nasty thing to be avoided at all costs and think you may be making your extension unnecessarily complicated. I would suggest that you speak to some local architectural designers, they should be able to provide free initial consultations and provide useful information about foundations, party wall act, neighbours etc. etc.


Good Luck

Little_Missy

Hi teneighty,

Many thanks for your response and the useful info.

Is it a legal requirement that the raft be designed by a structural engineer or, if I have the knowledge, can I design the raft and have it signed off by the building inspector?

If I was to use a raft foundation for a double storey extension, I presume I would have to make the raft deeper to support the extra weight?

Re. the Party Wall etc. Act 1995 - If an extension is built wholly on a person's own land, but against the boundary line (on raft foundations, so no foundations under neighbours land), can a neighbour still cause a dispute under the PWA resulting in the necessity to instruct a surveyor (or surveyors)? If they can, would building the foundations 1" from the boundary ensure that the works fall outside of the scope of the PWA?

Thanks

teneighty

If the extension needs Building Regulations approval they would certainly want to check your design calculations. I would suggest most people would need a structural engineer for that. (Different rules apply in Scotland). Apart from that the consequence of getting it wrong would be a cracked slab which would be very expensive and difficult to repair.


A two storey extension would probably need more reinforcement rather than a thicker slab.


The interpretation for building against the boundary (line of junction notice) varies between surveyors. I take the view that setting the wall back 25mm is fine. Others expect 50 - 100mm gap before they accept a line of junction notice is not required so it can be a bit pot luck. The adjoining owner can still dissent to a line of junction notice but to be honest it is really only a formality and it would take a very incompetent or dishonest surveyor to rack up any kind of fee on a small domestic extension case.