Constructing extension foundations

Hi all,
I am about to embark on building a small extension, of size 1.5m x 3.5m, sited on an existing concrete driveway. I've never done foundations before, and have heard that it is very hard work, so I wondered about getting someone in to dig and fill the foundations, and install a new rainwater gulley. I've had three quotes, 2100 (but only as part of a build-the-entire-extension quote which I didn't want), 2700, and 2900 (all inc VAT). They're roughly comparable so I guess they are probably fair prices, but they are higher than I'd anticipated when I first did the costings for the extension a year or so ago. Have prices gone up a lot recently?
To keep costs down, I am thinking about doing it myself, but I'm trying to work out whether it is worth the hassle. I think I've got the rough idea of what needs to be done, but a few questions arise:
- If I cut out the perimeter with a concrete saw, are gas/water services deep enough to avoid being cut, or must I locate them precisely first? I know where they are already to about +/- 30cm. - I will use a powered breaker to smash up the concrete in the centre, taking care to avoid damaging the services. Is a mini/micro-digger useful, or more hassle than its worth for a small site? Maybe a self- propelled skip loader is a useful compromise? - Will the BC surveyor want to see a test hole already dug right down to 1m, or will they hang around and watch a test hole being dug until they're happy it's deep enough? - If I bought in ready-mixed concrete, will the operators level it out for me, or would that be my job? - Any other tips, or recommendations for making the job easier?
I'm sure other questions will arise as I think about this more :-)
thanks,
dan.
, but are the savings really worth the effort?
thanks,
dan.
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dent wrote:

Water SHOULD be. Electricity SHOULD be. Gas I dunno.
Both SHOULD have warning tape over the top.
Bothe will be deeper than the concrete for sure, so cut concrete and then hand dig those areas.

Why? use it as a decnt sub base, put a DPM and insulatin down and screed over surely?

To be honest, I think if you are fit, you are making a bit of a mountain here.
Yes, hand digging is hard work, but the risk of damage is a lot less.
Also falling into the pit you have just dug with the digger, with the digger :-).
But a mini digger is fun and will do the job in an hour or so, BUT beware those services. Dead easy to smash a service with one., No feel.
How deep are you having to go?

The BC will want to see the complete trenches, and will advise on the depth to meet current requirements. If he is dubious because trees etc are around, he may also want you to get soil samples analysed.
If he is already involved, direct most questions to him. They are usually much friendlier when asked, than when being told...

Your job. Its not that hard. rakes and so on. And 'level' isn't that necessary. Simply lay an underground course of concrete block up to a decent level, using variable mortar beds of VERY strong cement (2:1)
However given the size, it may be just as easy to get a mixer and mix your own, which allows a micro adjustment at the final stage by adding little bits to the existing soggy puddle.

easiest way to get concrete more or less level is to hammer in pegs to concrete top using strings and levels, then fill to the required depth.
Ask BCO whether you need compressible polystyrene sheet to line the trench for heave..thi sis simply EPS pegged securely to the trench edges.
Ask BCO whether or not you can, if they are exposed, lay foundations directly round services. can't remember rules on this one. They may want them in a flexible sheath first.

Yes. Also you can adjust to taste, like using rebar to strengthen it
You may also decide to try and use bow ties or rebar to tie into holes made in existing foundations.

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