Stormwater Drainage System Rear Unit Development

Hi all

I'm currently building a unit at the back of an existing house
Its toward the end of the fixing stage
I've come up with a massive unexpected cost of 44k for the stormwater drainage system including builders 20% margin due to the the fact i didn't need a drainage plan drawn until the building permit. So the builders quote in the contract at that time didn't include this cost.

Attached are my Civil Drainage Plans

I am urgently asking for some help if any possible way to minimise to cost?

I have asked a few plumbers and there price is slightly cheaper around 35k but my builder stated that even if i source it to another plumber he will need to charge the 20% builders margin because his company will cover the 10 year warranty.
I have also rang the engineer that designed the plan said that was only best solution. I will ring another engineering company tomorrow to have a view of it.

Has anyone been in a similar situation with there development?

 

• Why is there no PC sum for drainage? Why didn't they include an allowance to drain to the street? Is it specifically excluded?
• Why is the markup 20% - is it in the schedule?
• There's a 2Kl tank but it will only hold 1Kl after slowly releasing the rest. There is only one dp charging it and it's reliant on a head of water to push the water into the tank rather than directly charging the tank, this will probably lead to your gutters overflowing.
• Is pit 3 required to be gravity or charged line (pumped) under the approval? It'll be much cheaper without pumps.
• The engineer has used the 450 dia pipe for storage is this a better option than a larger precast pit?

 

$35k seems quite steep for the work, of definitely shop it around. I also don't know about the builder putting on 20% even if you organise it, that sounds like a stitch up to me, the plumber has to guarantee it for 7years with his license so the builder isn't putting his neck out there.

I would speak to sober engineer and have him have a look at it, these are often over engineered and someone else might be able to streamline it a bit.

It looks like the engineer hasn't even verified the outlet height so he doesn't know if you need a pump or just a gravity fed system.
There looks to be a provision at the top saying if it can be gravity fed then the 450mm pipe down the driveway can be replaced with 150, that's a HUGE saving!

Depends on how hands on you are I would verify the outlet height and have some serious discussions with the engineer, possibly a new engineer and even council (all councils have different rules in terms of what needs to be ticked off by them or if the plumber and engineer can just tick it off)

I'm a drainage plumber and I do these detention systems everyday (eastern suburbs of Melbourne) so if you have more questions feel free to pm me and I'll try to help out a bit more.

 

@Raymond Dao - what's the comment?

 

I am not certain what the requirements are in Melbourne to connect the the council stormwater or what your soil conditions are. In WA there is no requirement to connect to council stormwater. The opposite is in fact the case, you have to retain storm water on site.

Had a look at your plans. In WA you would need no more than 2 x 1800mm interconnected storm water drains situated in the driveway where you have your 600 x 600 sump. Cost in WA $1800.00 supply and install for each drain plus the running of the pipes, maybe another $1,000 to $1,500.

Have a chat to a hydrolic engineer, they should be able to come up with a solution.

If the builder hasn't included the installation in the quote , do the installation yourself. Check your contract, but I do not believe you would be committed to using the builder for the installation.

 

Thats pretty shocking stuff that the builder wouldn't have a PC from the beginning. But surely it would have been a condition of the DA that you can demonstrate SW discharge...and you would have known it's not a cheap item to address....

 

Seems strange that the "quote" and subsequent contract didn't allow for stormwater drainage. Maybe go back and review the contract documents. The builder would have known there would be a SW connection - see if that's been allowed somewhere. Also, check the planning permit - see what conditions there were for SW drainage (they will normally be there) - did the builder have a copy? with a little luck he did and based the quote on that. You would have needed SW for the subdivision and building permit so it shouldn't have been a surprise at this late stage