Extension Foundation

[Lawrence Zarb]

I want to build an attached garage onto my semi detached house.

I live on the courner of two roads and have a largish tree on the pavement about 6ft away from the planned garage wall.

As I would like to have the garage wall act as a boundary against the pavement I will have to build the wall on the edge of the footings.

Also I have a high water table and clay soil.

I beleive I will require a piled foundation.

Who do I need to speak to to get an estimate/design of the foundation.

Would anyone have any idea of the cost of this foundation for a 7m x 3m foundation (only 2 sides required)

[Weatherlawyer]

Start with whoever it was that told you about piled foundations.

You have three major obstacles: the tree, the soil and the traffic. Search the net for resolutions to these individually:

Nearby trees and foundations

Foundations traffic

Foundations in clay.

[Phil L]

Foundations in clay are perfectly acceptable, we live in an area of clay and everything is built on it.

To the OP: Piles are much more expensive than strip footings, so don't go down this route unless it's impossible to avoid, you can apply to have the tree removed, although success is not garuanteed.

The first thing you need to do is get some plans drawn up and submitted, then dig some test holes for the BCO to decide whether you can build on strips or not, if he says it has to go on piles then your plans will need to be altered and accepted by BC.

I've never known the water table to affect footings whether they are on strips, piles or a raft, nor traffic.

[The Natural Philosopher]

Agreed. I was looking at those, but in the end a man with a digger going down over 2meters was enough to cope with some serious trees.

A raised floor to account for any heave underneath teh site coped with slight movement.

Its an advantage with clay..the biggest problem is massive shrinkage, followed by heave after rain. Permanently soggy is better.

[Phil L]

Shrinkage often occurs after tree removal, although I'm not sure why, some say it's the mass of smaller roots rotting away and leaving voids, some say it's the clay's water content changing...either way, tree removal is best left to the profesionals, I know of one bloke who recently cleared a row of Lleylandii from his boundary in order to build an extension...the removal partially destroyed the main sewer under the road adjacent to his property, end result, £36K to get it repaired and no extension.

[Guy King]

The message from "Phil L" contains these words:

Bastards to dig though. Clay isn't the nice soft stuff you get in nursery, it's hard as nails.

[Lawrence Zarb]

I want to keep the inital costs down, just in case its not practicle to have this built.

If I dug the test holes wood the BCO attend site without any formal applications to advise what is acceptable?

[Phil L]

I prefer clay :-p I dug out about 12m3 of sandy soil for an extension in a nearby town a few months ago...it wasn't too bad lower down where it was compacted, but higher up where it was loose it was a PITA, it kept caving in - the trench was supposed to be 600mm wide but at the top it was nearer to a metre...the last one was in clay but the trench was 23m long, 1.2m deep and 600mm wide :-( the digger man did it in 2.5 days :-)

[Tony Bryer]

It's a street tree, and I would doubt that the average Council would be willing to remove it so as to make constructing the OP's extension easier. And given that we are presumably talking about fairly deep excavations along the back of the pavement, there may be issues of maintaining support etc, which make me think that piling may be easiest in this case.

[The Natural Philosopher]

No. *Heave* occurs after tree removal..at any distance..The lack of water drawn allows the moisture content to increase.

Shrinkage is the issue only where the bulk root mass is.

[The Natural Philosopher]

Not at any depth its not.

[The Natural Philosopher]

He might,or get a structural engineer in - the BCO is not actually that qualified - best is to contact a firm of engineers,or ask BCO who he analysed samples ate depth to establish root hair content. They went down 1.5 meters so 2 meters it was...

[The Natural Philosopher]

Its usually a straight cost benefit analysis. Sometimes piles are cheaper, but mostly when access is good, in clay, they are not.

[Lawrence Zarb]

What sort of money are we taling about to get a structual engineer to take a look?

[Phil L]

The first thing you need to do is get plans drawn up and submitted...you /can/ do it on a building notice but it's not advisable, having plans draw will probably cost around £400 - £600 but this will probably include submitting them as well, which is around £150...look in local rag for 'plans drawn'. If you do it on a building notice (without plans) and you come across problems like piles being required, it throws it all to c*ck, whereas plans can simply be amended and work can continue, the person who draws the plans will probably indicate strip footings etc, but be aware that you probably won't be allowed to build right up to the boundary, because the foundations will be 600mm wide and the wall sits in the middle of this, meaning you will be digging under the pavement.

[The Natural Philosopher]

couple of hundred.

[Matt]

Stripping the bark off round the full circumference and a few feet high will ensure rapid removal by the council.

[Aidan Karley]

And potentially a criminal record, as well as the cost of replacing the tree.