First, if your plot is less than 0.045Ha; it will not be approvsd for flats in Kiambu County for flats construction. Late last year the county started enforcing this requirement. Hundreds of developers in Ruaka, ndenderu and muchatha areas were caught offguard. These are some of the issues making Clifford popular for Kiambu governorship

@Thirstforknowledge the process stands as below:
1. Engage an architect. As the main planner of the project, he has to come up with a suitable design, both aesthetically(kwa kuona na macho) and CAPEX-wise(kulingana na uzito wa mfuko). Once a suitable plan is decided upon, it is supposed to be submitted to the local council (in your case,kwa kabogo) for approval. They usually indicate whether the plan will be approved or thrown back to the drawing board based on what @kanyimwa has indicated, amongst other things. e.g. number of floors, zoning of the area (low density residential, high density residential, commercial,etc).
2. Upon approval, you engage a structural engineer. From my experience, it is better to engage one when you engage an architect, and you would also discover that architects like to bring in their engineer for purposes of ease of communication and previous experiences in design and supervision works in the same team. I advocate for early appointment of the engineer as he would guide the architect on some structural aspects before the architectural design is finalised, and thus ease in undertaking structural design as well as eliminating any problems that may arise pre-construction.
3. Engage a quantity surveyor(QS). He is the projects cost estimator and gives a preliminary cost of the project. His importance will also e stated somewhere down this summary.
4. Engage a contractor. This days, the NCA makes it mandatory to have a contractor registered by them undertaking construction works. The architect, engineer and QS usually have a hand in vetting the above in the technical aspects of construction projects undertaken before by the prospective contractor, the technical people in their team, among other items. you will also notice that most contractors these days only bring machinery and materials on site, then hire a labour sub-contractor to deal with manpower.
5. Register the proposed construction with NCA. They usually require approved architectural and structural drawings, as well as the QS's estimate to determine your 0.5% levy payable to them. Without this, you will not get a go-ahead for construction. NB: the 0.5% levy is applicable for projects costing 5 million and above, or thereabouts.
6. Fees payable to the consultants:
Architect:4-6%, Engineer(Structural, Mechanical and electrical; each): 3-4.5%, QS:2%.
Some consultants may offer a rate lower than what I have given,depending on your negotiation skills.
I think I have covered all the bases.

NEMA wasipuuzwe

Where does project managers come in and would it be cost effective to engage project managers ( like the Mentor Group )from the onset of the project?

Weka 40k for the EIA expert. Plus 90k for change of user as demanded by cap 286 of the physical Planning act. On administrative cost alone weka 400-500k in budget as per accurate list availed by @musimo