Thanks Pablo:
Looove the diverse opinion of Wazuans!

Here is what I am thinking of doing, which blends what you suggest and what Ngong and the Captain suggest above:

1. Have the fundi estimate labor costs for the whole project up to plastering. Electrician, plumber and roofer would give me their separate estimates.

2. Chop the project into smaller but complete phases (e.g Koroga/ground floor slab, walling, stairs and first floor slab etc).

3. Have him estimate labor costs for each of these discrete phases separately.

I would gauge whether his estimates are reasonable based on the costs Ngong gave me. Each of the phases would have its own payment schedule. If we disagree at any stage, we can part ways after a phase is done.

BTW he comes vetted. I have spoken to some of his former clients and they seem happy. I am planning to go see some of his handiwork pretty soon.

More alternative thoughts welcome.

1. "I am trying to teach myself how these things are done..." if this is your genuine intention,you have to be on site so no need for head fund earning extra. pay him as a normal fundi let him assemble his team.

2. Fundis over quote on Labour contracts, use daily rates as the basis but ensure they know what they have to deliver daily/ duration of tasks/phases. Pablo has a formula of 1 fundi does 120 ft/day ensure that they know this and if it falls short kata pesa!

3. Never get too firendly with fundis on site i have found that erodes their repect for you and affects their performance

I have done several constructions for my self but every time I contract I get alot of unnecessary conflicts.
One almost turned tragic.
It happened I got a good fundi from my friends site and he told me the best thing is I contract him labour and he will do a good job and there was no doubt about that.
So every time I paid him as agreed he will go ahead and pay his crew half amounts and claim I havent given him enough which was a lie.
So this saturday I had a sick person who I had to discharge from hospital,the process to longer past 7pm.
I had to pay 1000 shs, to be served at Equity bank upper hill so I get all his cash.I met with him as agreed at a point and gave him all the cash.
On getting to my compound at around 9 pm night, I found all the other fundis and WWM waiting for me with faces that suggested this is a final game!
I asked them what the matter is and quickly said tupe pesa zetu.
I asked again which money?
The told me niwache upumbavu and I should not waste their time they have some important matter to do.
I told them I owe them nothing since I have given cash to the fundi.
They said I am a liar since I always give tokens.
Luckily I had my sick person with me who told them I have actually given out their cash.
They left promising to come back since the fundi had switched his phone off.
So they headed to the fundis home some nyumba za mabati about 20 houses or so.
When they knocked his door,he just shouted Wezi and his neighbors came out with all kinds of weapons...

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@Dahatre,
my submission would be a huge %age of the labour cost is absolutely determined by the model you opt to take e.g if you are naive in matters construction my advise would be to contract the labour in phases i.e how much for foundation? then ground flr walling, shutter, 1st flr walling, roofing etc. it will save you lots of pain.
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I agree with this. I just completed building my mum a house, my very first project. I used a cousin to supervise and he contracted fundis in phases. I am surprised that building is not so expensive. I am all for phased contracts.

i am going through a similar challenge, will you be supplying the tools also like a cement mixer wheel barrows etc or will the fundi?