Floor - concrete slab vs block & beam - COST

[Dankat]

I'm after some advice from some experts out there.

We are looking to have a single storey rear extension completed on a semi-detached house - 5.8 metres across and 4 metres deep.

When choosing the floor construction, is there a simple way of working out the relative cost of concrete slab vs block & beam? Are there any websites that allow you to calculate a price per square metre for the two methods? Would there be any variance in the foundations used for each method?

Site access is via a path down the side of the house approx. 90 cms wide. Property is in South Central England.

As an aside, the soil is clay based and there is a large oak tree that will sit around 15 metres away from the extension.

Any thoughts would be appreciated.

 

[Notch7]

It is more expensive, materials are a bit more and it is more labour intensive.

If you existing house foundation and construction is good then pockets can be chopped out and the beams can then be supported 1 end by the house. The beams would then be 3900mm.

I would be guessing around £650 or so for the beams, £300 for the blocks (incl vat) You may need extra soil removed as there has to be 150 min clearance below beams for ventilation.

A solid floor would need, type 1, sand blinding, concrete oversite

Given the tree an soil type, I would say the building inspector may insist on a suspended floor.

No difference to foundations, but bear in mind the oak tree will mean deeper footings (check on NHBC foundations with trees calculator to get an idea).

 

[Dankat]

It is more expensive, materials are a bit more and it is more labour intensive.

Hello Notch7.

Thanks for your response, but I'm not sure which of the two you are saying is more expensive for a project our size?

 

[Notch7]

Oops! apols for the lack of clarity!

the block and beam is dearer -more on materials more labour to fit.

 

[Dankat]

Thanks Notch7

Would the price difference between the two be substantial, say if it were measured in percentage terms?

 

[Notch7]

Thanks Notch7

Would the price difference between the two be substantial, say if it were measured in percentage terms?

I would guess at £1,700+vat block and beam compared to £1,100+vat for concrete slab oversite allowing for labour cost and margin for building contractor.

Note that reduced dig (ie digging out ground to allow for thickness of floor make up), skips and labour not included but same for both floor options. Plant not included either, although a digger etc will be on site anyway for founds.

If you are having carpet, timber or similar floor then you can have a timber suspended floor instead as an option and no screed drying time. Ive done this on jobs for various reasons. In fact I have a job that was to be concrete slab but is now going to be suspended timber with underfloor heating which is cast in a 30mm screed layer for efficient heating as it will act as a thermal store. It will require oversized joists and insulation in between and over the joists.