Site Works and Preliminary Cost Estimates

So I'm building a two storey house on my 342m2 plot of land. The soil is not great (a bit of wet clay) - see below, and the recommendations if I'm interpreting correctly are an H2 waffle slab with screw piles, or a stiffened raft slab.




I've got several builders to give me a quote, and I'm worried about the huge differences in site works and slab costs quoted.

One of the builders which we had paid a initial fee for consultation and design has given me their "accurate and final" total cost of site works (establishment, ground works, slab, plumbing) of $64900+ $52800+ $34900 = $152600. Which I thought was taking the mick just a bit.


So I've been to other builders, talked through the project and shown them the soil tests above, and have had preliminary quotes (or allowances) of



and



whilst another has suggested it would only be a $10000 upgrade from their normal M class slab.

So who do I believe? The latter quotes are preliminary quotes only, and I guess my question is how accurate do the preliminary quotes have to be? If they tell me one figure, and then I pay the deposit to go to engineering and tender, and they turn around and say "sorry mate, the preliminary quote was completely wrong, it's actually another $100k for all the site costs, do I have any comeback on that?

Any feedback gratefully received.

They simply can't perform any "accurate" calculations on slab and plumbing without at least draft structural engineering and stormwater drawings.

If your house plan is finalized, why don't you do your own engineering, storm water and BASIX first and then just obtain 3-5 quotes from relevant vendors, specifically earthmovers, concreters, stormwater/drainage plumbers.

Those quotes will provide you a very strong edge when discussing with the builders.

From what I see, your first quote can be up to 300% inflated of what the real costs will be, but again, makes sense to discuss once the engineering is done.

You think I should get my own engineering done? I thought that would be doubling up on things. The builders all seem to want to do their own engineering, with someone they know and trust. Even the soil report I gave them, they said they would do that again to confirm what slab they would do.

I guess that would certainly give me a good estimate of what the siteworks should be though.

But even so, the builders are free to quote whatever they want, aren't they? They certainly don't have to take heed of my engineering and estimate...?

Well, it is either one way or another.

If the builder's engineer will be having doubts about the quality of your engineering, the builder can always get it re-done (and charge you as he would had charged you anyway), however, the overall savings may be significant. If you just ask for slab/footings engineering only, it will be costing may be $1.5-2K tops. You will be paying more for your tenders for sure.

On the other side, may be you could just ask some structural engineers here, e.g. StructuralBIMGuy on what will be their recommended approach to your slab and based on that, estimate relevant costs.

Your block is relatively flat and building envelope is not so huge, concrete prices in VIC are currently relatively low, your slab should not be costing a fortune even with piers, etc. $52K is definitely way too much.

Also, how far are your services connections? $35K for drainage plumbing/services alone seem to be ridiculously high as well.

That a general misconception,
Society ( Trusted Professions ) TRUSTS Engineers They don't necessarily trusts Builders (as they haven't made the cut)
That trust Should not be transferable...Builders have to earn your trust..get the gist
Use your Sketch & Engineering Drawings in selecting a builder, get 3 quotes, compare,etc

So avoid those builders, Armed with your engineering Building Information, Data and proofs, select from others

I asked him why the site costs were so high, and he said the concrete prices are very high at the moment LOL!

My service connections are very close by. Indeed, I think the whole quote is ridiculous. $64,900 for engineering, finishes schedule and OHS. I gave them all the working drawings, I've done all the temporary feencing, and I did the VicSmart application for them already. And this is one of the well known "reputable" custom builders in my region too.

Thanks for the advice StructuralBIMGuy.

I'll take it on board. I think I'll initially leave it to the builder, but if they quote me something that just doesn't sound right at all, then I'll enlist the services of my own engineer to quote the slab and structural costs and see what they say. Perhaps I'll have to look for your business, if you're practising in Melbourne.

ben888 the prices I am seeing coming from the East coast are extremely high.
So you will need to look where you can save money on your builds. Forewarned is forearmed
Here are some points to consider
1. In order to check prices & Alternative solutions, that you can understand you need Building Information (3D Models+ Data) ie Quantities Rates etc
2. Builders & Designers Use 3D Models for marketing coloured images ie how it will look? They will never give you Data, nor do they understand the math/calculus
3. Engineering BIM uses data and proofs whereby allowing you to Optimize your build while assisting you in the decision making process
4.Most engineers are fairly conservative & They may not want to disrupt the Status Quo.. find one one nearby that will
5.New home salesman, Architects/Designer earn high commissions/ margins...they profit from high price builds... you just need to know what's a fair price then you decide
OT, I am not here to profit but here to assist.. The irony is I choose not to have a website yet google & Others rank my community participation highly

Lol, cost of concrete itself creates a small difference.

E.g. if you have 30 m3 of concrete in your slab and you pay $300 per m3 (which is on a very high side in NSW), you pay $9K for your concrete vs $6K if you pay $200 per m3 (and I have heard you can 25 Mpa for $180 m3 in VIC those days), so the whole difference is only $3K.