Andrew Welham wrote:
Dear ALl

My request is about foundations for an extension I am planning, at
this stage I am just looking for pointers so I can have a sensible
chat with the building inspector, prior to creating any plans.
Sorry if this post is a little long, but I need to explain my ideas
A picture to illustrate this conversation can be found at the
following address

http://homepages.nildram.co.uk/~awelham/Foundations.gif

I am looking to build a single story extension in the shape of an L
round my house.
I want to build right up against my boundaries (which are shown by the
thicker lines along the sides of each drawing

The picture on the left shown my ideas for foundations and the one on
the right shown the room layout (excluding external entrances)

I want to do this building three phases
- Room 1 first
- The new garage
- Room 2&3

This is for a number of reasons, including breaking the build down
into three separate phases, and most importantly there is currently a
garage where room 2 is going to be located.


I want to build right up again my boundaries and I understand that I
can’t build using trench foundations as if I don’t build in the centre
of the foundation they will rotate over time. For this reason I am
considering slab foundations for the part closest to the house and
then traditional trench foundations on the other side of the drain
pipe. I understand that slab foundations are usually at ground level
as the slab also forms the floor of the building, how ever in my case
I want finish the slab at the same level as the top of the trench
foundations, and I will them create a raided floor over the entire
area. This is not only warmer, but also created a nice place to
locate pipes and cables.

Have you recently won the lottery?

To bridge foundations (over the drain pipe) I am considering using
lintels (The strongest possible). I am looking to building above
building code where ever possible to ensure maximum strength in the
building.

The 2nd Slab would form the garage floor with a single wall no
insulated. I don’t intent to park a car in the garage, but instead
use the space to store my tools etc. For this use is a garage the
best designation for this type of building?
There would also be short walls to form the back and front of the
garage.
Finally comes slab 3 & trench 3 used to form room 2. By completing the
wall in trench 3 then room 3 will automatically be built.

Now I have explained my ideas I have the following questions.

1. Is a garage the best designation for the use described above?

I don't understand this question.

2. Confirmation I can build on the edge of a slab?

By 'slab' I presume you mean raft? - why are yopu contemplating using a
raft, then using block and beam floors over the top, coupled with trench
foundations at one side?

3. I am a little concerned especially with the garage ahat all the
load will be on the wall opposite the house, is this still ok, ??? or
will the slab rotate ?

If it ends up going on a raft, the raft will cover the entire area and will
also incorporate a 'toe' - a huge concrete counterbalance on the opposite
side.

4. Should I be using any additional additives in the concrete? I have
heard of some fibres which are stronger than re bar, is this correct /
advisable?

The person who designs the raft will also design the steel that goes into
it, it will need to be mechanically shaped, bent and sometimes welded into
the shape of the concrete which is to be poured - all the concrete has to go
in on the same day - you can't do it in sections.

5. Can I mix slabs and trench foundations in the same build? (If so
what do I need to consider) re bars????

see above

6. Can I build a trench foundation in a number of distinct phases i.e.
trench 1 & 3 which will eventually form a single trench, should I use
re bar to ensure the concrete strength is retained along the join?

You can but they don't like it, it isn't as strong as one long trench and
one continuous run of concrete.

7. Is what I have described a recommended method? If not what are the
alternatives

Just about everything you have thought of so far is the most expensive, time
consuming and outright unnecesary

8. Is there any other way to build right up against a boundary without
impacting the neighbours land?

Build on trench footings, you may lose six inches, but this raft will cost
you at least 20 grand more than strips

9. Any concerns the building inspector would have when they see this??


He'll tell you to cover the drains which run through the proposed building
with concrete, and he means they have to be completely encased in it...what
would be a thousand times easier would be to dig your back trench twice the
width and run a new plastic drain from one side to the other, then connect
up to the two existing drains at either side thn the drains are not under
your building at all, extra cost = about £150