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.