If your engineer is truely an engineer, do not ignore or engage in tussles with him/her at all. Being an engineer is not only paper work alone, its experience.

Clay soils occur due to poorly drained areas, this means a hard formation might not be too far from the ground. In this light, do not compromise on the safety of your structure.

Use of raft foundation is very expensive for a 1 storeyed building, unless you intend to use the building as a parking for many vehicle or warehouse for heavy equipments and goods.

I would recommend use of pillars with pad footings and ground beams i.e. beams connecting the columns/pillars on the ground that can assist in ensuring that incase there is any settlement of the foundation material, the settlement is uniform.

Another way is to entirely remove the black cotton soil and borrowing red coffee soil or murram and compacting it well, then ensuring that the plot is well drained such that stagnation of water does not occur hence 'rotting' the soil. This is by far cheaper that rafting!

What? 22ft. Kwani its where? 22ft is 6.7 meters, i.e. 2No. floors!

@ Injere
Karibu Jirani. Did my pillars/columns at 14ft, the numbers changed drastically and had to source for alternative funds to complete the structure. No regrets tho. Either way; dont give up, your dream home is only a few miles a way. There is a neighbour who only used pillars/columns but no Y8,10 & 12 on the foundation, only BRC. He is now roofing. Wonder for how long the house will stand if its true.

at 2700 floor - roof , 6700 is slightly more than 2 floors even with addition of slab depth. Could injere explain what kind a building he seeks to put up ?

I was also shocked to see so many structures coming up next to mhindi slaughterhouse.Those plots,were they sold by Njiru Housing Company which an office on site.

Probably i have misled you guys - it's actually black cotton soil plot and not clay soil..... and VERY far away from Njiru!!It's off Thika Rd actually.

Let's keep to the topic though: Those who have done the workings or have some experience with this- which is the cheaper alternative given this kind of bed rock depth and is any of the two technically superior?

Refer to post no 7.

The engineer may do a design for both and cost them accordingly in line with the loading of the structure. Only then can which is better(economical) be determined. No quick answer to your query in my view.

I hope by pillars you mean piles, the kind being done by Chinese to support the overpasses on Thika road. Piling is expensive and the few contractors available charge highly. Your Engineer will make a decision based on the soil bearing capacity . I don't have much data but it seems you'll go the raft way.

The panacea to your issue is to follow your engineer's advice. Looks you'll have to invest heavily on your black cotton plot. If it were possible it would be better if there was a floor down there... whats the point of excavating and then filling it up. But then you'll have to contend with flooding.

For me I would go the pillar way with suspended structure,,,.. but don't give up