Concrete slab under a pole house

[ryanstev]

I've bought a new house, moving in on the 13th of December.
It's a house that's on a pretty steep slope, one end of the house is on the ground, the other end is on 3 meter high poles.

Unfortunately the previous owners decided to dig under the house, further into the bank, they've almost completely exposed 3 footings for the poles and partially (probably 50%) exposed the next 3 poles behind it.

I bought the house with plans to have a concrete slab poured underneath and put in a garage and two rooms.
I'll have a cinder block wall to replace the fully exposed poles and fill in the bank between the wall and the bank to cover up the partially exposed footings. I'm just guessing that I can do that, otherwise I'll re-pile the other three poles, or possibly just extend the downstairs to them.

I have a fair idea of costs for the concrete slab, it'll be 7m x 7.7m, so 54 square meters, from searching this forum, it seems that I'd pay $130 per square meter, so $7,000 for that.
Considering none of the walls will be load bearing, I'm guessing that the exterior walls will be cheap to do, just like the interior walls.
I'd like to have internal access, the lounge is directly above, so I'll have a staircase put in.

So my questions are really only about 2 things.

1. How much would you expect the building consent to cost? The house is in West Auckland.
2. How much would you expect the exterior walls to cost? The cinder block wall will be 7.7 meters in length, it'll act as a retaining wall (assuming this is allowed) the other three walls will be 7m, 7m and 7.7m, with a couple windows each and a single garage door in one of the 7m walls.
The cinder block wall will be around 2m high. (Actually, a bit lower, depending on how thick the concrete slab is)

I've been guessing how much this will cost by looking at quotes I had for Versatile garages, they were similar in size and I was quoted $25,000 for one, but this was a freestanding building, so load bearing walls and a roof.
My building won't have those, so I'm hoping it'll cost $18,000 in total, or less. (Plus the cost of the slab)

Ultimately I'm going to get quotes from some builders, but with the holiday season upon us, that's not going to happen until sometime next year and in the meantime I'm very curious as to costs.

You can see a picture of the area here:
kaitaia.com/foundation.jpg

[Davo36]

Mate, I'm in west Auckland, and we had this done around 4 years ago. Was a bigger project and cost $160k. Our area was approx 75 sqm.

You are totally dreaming with figures like $18k. You'll spend more than $10k before you start. Plans, geotech reports, building consent...

Things like Versatile garages are not in any way comparable.

Don't want to put you off, but for an area 7m square, you'd be looking at $100k to do it nicely.

[ryanstev]

Well, that's bad news.

My parents had something similar done 10 years ago and I recall the first part cost them $40,000 but they had plumbing and the building extended out from under the house by a metre or two, all up it cost them $80,000 but they had to make structural changes, as the original outside walls were load bearing, as well as the poles.

My initial idea was to pour a concrete slab and make the entire downstairs a garage, but when measuring it, etc, I realised I could easily fit a couple of rooms in.
Maybe I'll go back to that idea, just have a slab with 4 walls and make it a big garage, external access.

[elguapo]

You are totally dreaming with figures like $18k.

Don't want to put you off, but for an area 7m square, you'd be looking at $100k to do it nicely.

Yeap, I'd say $100k is a realistic starting point for a budget. Even if the solution is simple, you could very easily spend tens of thousands in the design, engineering and consent process.

[ryanstev]

Well, I suppose my next questions are:

1. Is it around $7,000 for 54 square meters of concrete? It'll be reinforced and have the water membrane.
2. How much would you expect to pay for the cinder block wall at the back, which will be load bearing for the house and (again assuming it's allowed) a retaining wall for the bank?
3. How much would you pay for the exterior walls? They'll be done cheaply, so the inside of those will have exposed studs, etc, no gib over them, as it'll just be a garage.

Thanks for the replies.

[Wayne]

I had to look up 'cinder block' to check we were talking about the same thing - an American term. We call them concrete blocks.
Anyway - the cost would depend strongly on the situation.
How much room to lay the block, and get behind to seal, provide drainage and back-fill.
The blocks will need to be reinforced (steel and concrete) so that adds to the cost.
I expect you will also need an engineers design.

You may want to make cheap walls but they still have to be 'per code'. So a lot depends on the clading you are talking about.
You will still need building paper and maybe a cavity etc.

54sqm of concrete slab would be 5.4cum of concrete at $225/cube so $1200 or so. I would have expected change out of $7k for laying etc but don't really know.

[ryanstev]

Thanks for the replies everyone.
I know I'm asking "How long is a piece of string" questions.

[skid]

I would have thought what the previous owners did would have compromised the structure of the house.
Did you get it checked out before buying?

[elguapo]

Well, I suppose my next questions are:

1. Is it around $7,000 for 54 square meters of concrete? It'll be reinforced and have the water membrane.
2. How much would you expect to pay for the cinder block wall at the back, which will be load bearing for the house and (again assuming it's allowed) a retaining wall for the bank?
3. How much would you pay for the exterior walls? They'll be done cheaply, so the inside of those will have exposed studs, etc, no gib over them, as it'll just be a garage.

Thanks for the replies.

1. Reinforced concrete is typically $1,000/m3 completed incl steel, shuttering, stripping etc. $7k isn't too far off the money.
2. For a structural retaining wall you will need an engineer to design it, that will be a few thousand.
3. Not very expensive, but you might find some headaches getting the cladding system tied into the existing structure. Anything to do with cladding in an existing house these days seems to invite weird and wild quotes.

[ryanstev]

I would have thought what the previous owners did would have compromised the structure of the house.
Did you get it checked out before buying?

Yes, the building inspector checked it.

In the picture I posted you can see some thick poles with big steel girders on them, they're very beefy the steel girder stops just over the first set of exposed foundations.
The inspection report said there was no evidence of movement, so I think it'll be straight forward to fix or replace those.

[Maccachic]

Was the inspector qualified to comment on the footings?

[ryanstev]

Well he told me there was little to no movement, but referred me to someone else who visited and checked it out.
He gave me a rough estimate of $1,450 per pile to re-pile and said it would be pretty straight forward to remove one row of piles and put the concrete block wall up to replace the other row.

[sidinz]

Is a building consent required to repile? If yes, does it matter if existing piles are failing?

[donna]

Here is the visual and if you click on it - it will open into a larger image.

cheers,

donna

[ben9465]

The movement the inspector is referring to is probably sinking or vertical movement. The problem with the basement dig out is that in an earthquake or hurricane the poles may not have the lateral support of the ground to stop the house moving sideways. You can get away with all sorts of shoddy construction situations until a major test comes such as an earthquake, tornado , hurricane etc and then these things expose themselves. The ctv building in Christchurch being an example. Any construction activity of a more than minor nature is very expensive these days. You will need council consent $2000-$$3000, plans $2000, engineers calcs $1500, excavation $3000, steel reinforcing $3000, concrete pumps $600, concrete $1500, block fill $800, Blocks $200, builders labour $8000, blocklayer $1000, concrete finisher $800,waterproofing etc etc etc . It all adds up, often the main material used is just a small component. The last house I built had about $10 000 of timber out of the $150 000 I spent on it. So Probably $70 000 + is quite fair estimate.