Foundation cost

We are building a house with a building company and have a 290sq meter foundation. After doing ground works they need to make foundation 100mm deeper and say it will be another 5k.
Does this sound about right?

bunny2121, Aug 6, 9:21 am

I would ask why they needed to make it deeper? Also why the cost was not advised before hand? Also an invoice of the additional costs

smoff79, Aug 6, 10:03 am

I'm guessing they had to redesign the foundation because of the nature of the ground they found? $5K is for the re-design, council approval, additional build cost?

Seems reasonable. But I would still make further enquiry to satisfy myself of that.

pico42, Aug 6, 3:05 pm

The plans would say foundation depth of 400mm, after digger dug out dirt and engineer been to test, the sub ground may have been a little soft still so engineer says take 100mm lower.
So extra 29m3 to transport away and extra hardfill to buy and transport in. 50 tons of extra metal, digger and man hours to compact and builders margins, 5k sounds about right.

kiwi_crusader, Aug 6, 9:24 pm

Ah, so they just removed unsuitable material and replaced it with suitable hard fill.
Get a breakdown of the cost - quantity x cost of each item (eg machine, men, fill etc.)

pico42, Aug 7, 8:25 am



Tell me how you do a Geotech report on foundations before there dug out please. A Geotech report is also not required for building consent. Also a Geotech report is the responsibility of the land owner not the builder.

underconstructy, Aug 7, 1:43 pm



When we purchased our section 18 months ago, all of the sections we were looking at had a Geotech report included for the purchase price. I would never have considered purchasing a section without a supplied Geotech report, otherwise how would you know if it was TC1, TC2 or TC3? Maybe you need to step away from the computer as the advice you are giving is extremely misleading.

kew, Aug 7, 5:26 pm



Hi bunny2121. I feel for you as we had a similar (although more severe) situation. Any chance that you can name the foundation company? I might be able to help/

kew, Aug 7, 5:35 pm

It's very clear you guys don't understand what a Geotechnical report is, what information it provides or how it is carried out.

Tell me how a Geotechnical report determines the ground bearing capacity for foundations.

underconstructy, Aug 7, 9:54 pm



Wow if you are doing any construction in nz please go back to polytech because clearly you missed the basics. The geotechnical report is required for consent and should be read carefully in quoting a job. It gives a good indication of suitable bearing that then needs to be confirmed by a engineer once the founds are dug at which stage the engineer may ask for some soft bits to be removed. I hope your building experience improves for the rest of your build op I imagine it's really scary when costs increase at such a early stage of the build make sure you are satisfied before handing over any extra money

lee289, Aug 7, 10:32 pm



And another who doesn't understand a Geotechnical report.

underconstructy, Aug 7, 10:36 pm



Not me that has the lack of understanding chief quoting is a challenge even for the most experienced makes me feel really sorry for people getting sucked into what they think is a fixed price just to have some con artist crank the price up for often unjustified reasons apart from they either mucked up the quote or underpriced it to get the job. It's defently a good idea to get three quotes and pick the middle price if it's to good to be true it probably is.

lee289, Aug 8, 8:34 am



Geotech reports are required in the Chch Area for a building consent on a house .

martin11, Aug 8, 8:37 am

underconstructy is right, geotechs will only test 3-4 spots around the site and use this to conduct their report, nothing can be confirmed until foundations are dug, geotechs will then return to confirm suitable ground bearing, too many times I have seen geotechs recommend to excavate up to 600m, then once the build has started they end up digging down up to 2 metres, it is naive to suggest the builder is liable for these cost, they don't own the land.

cortstang, Aug 9, 5:55 pm



That is correct. However if it is discovered at the time of excavation that additional ground needs to be removed, this would involve gaining an instruction from the engineer, along with advising the client of additional works and associated costs. The builder can't just go ahead and either remove, or claim that he had to remove an additional 100mm

kew, Aug 10, 7:42 am

We have a fixed price build with our building company. There are also a list of variables. The dig out for the foundations is not on that list. We have been told that if they have to dig out more than expected, that we will not be charged any extra. I guess that goes both ways though. We are on TC3 land.

blb71, Aug 10, 7:48 am

no geotech did testing on my soil it was the engineer and he did pentromiter (sp) and then advised the depth and load capacity of the ground. One assumes that if this bit did not stack up then it would have moved to a geo person. we needed it for building consent. but the fact is the building company should have had this done before it went into council so effectively all costed up. I would again ask for complete costing of what was required and the 5k came about

smoff79, Aug 11, 12:13 pm



Why would the builder engage an engineer or even a geotech? They should be provided engineered drawings from the owner or designer to price the job from. If the builder is not provided all relevant information what do you expect?

underconstructy, Aug 11, 12:27 pm

did not come from builder it was required by council and sent to him by founds designer and by architect to be done they required the pentromiter test on the ground to be done to confirm slab was ok as designed. one can assume a builder would have all this info to cost it up before signed on dotted line. mine did.

smoff79, Sep 10, 3:23 am

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