In Kenya, land is zoned by local authorities and given a set of by-laws that govern how it will be utilised. When buying land with the intention of developing it in future, it is important to seek clarification with the area's local authority first so as to find what is allowed for built areas in that location.

In Nairobi, for example, the City Council of Nairobi allows for only one single family house on half-acre plots in upmarket estates such as Runda, Muthaiga and Karen. The same local authority does allow subdivision of land to a minimum eighth-acre in areas where the number of family units can be increased. That is, allowing high-density buildings such as apartments.

Topography

Land on sloppy areas, such as at RedHill or Murang'a, poses a challenge when it comes to laying down the foundation. Such areas require a lot of excavation and need soil-restraining walls. If the cost is not captured in the budget, then this could lead to project cost overruns.

However, slopes have the advantage of being well-drained compared to flat areas such as Ruai, Mombasa Road, South B and C. Flat areas tend to flood in heavy rains, and because of very low water tables, the foundation is usually more expensive, such as in places like Runda and Membley in Ruiru.

It is important to look out for physical presence of temporary water bodies such as streams, rivers and lakes because such areas risk floods when the rivers or streams break their banks. Presence of papyrus reeds and long grass may signify periods of flooding. Dry valleys may be proof of temporary rivers.

Narok and Naivasha towns are a good example of flat land that is prone to flooding in rainy seasons. In these areas, ground floor slab should be higher than ground level.

Soils

Constructing on Black Cotton soil presents unique challenges. These soils tend to swell when it rains and contract in dry seasons. This expansion and contraction can push the floor slab upward or downward, creating cracks on it. This requires deeper overhangs and wider external pavements to ensure rain water is kept off the building. Heavy gauge polythene will be requisite to waterproof the soil from the interior. A minimum depth of one foot for hardcore will serve to soak in water and keep the soil dry.

Red soil is stable and does not change in volume when wet or dry hence will cost less to construct on it.

So, if the contractor uses some ingenuity, he or she would save a lot on costs. Simple solutions such as lifting the slab slightly higher from ground level, waterproofing the soil in the foundation and increasing the cement ratio on the ground floor slab can save what would otherwise be spent on excavation and backfilling the foundation.

Gichuhi is the principal consultant at A4architect.com.