We would consider those with lands at Arepo, Ajah, Sangotedo, Ibeshe, Ibeju Lekki axis and other areas (in and outside Lagos) where water level is only about 1m or less below ground level.
Normally during raining seasons, these lands are usually waterlogged making it almost impossible to work., in-fact, buildings are better erected during the dry season but I'll advice during any of such raining season, take time out to study the behavior of water on your site and around it.
The few things to note are:
A. Which direction water flows from or to.
B. How high from your present level it rises to, i.e the highest water point during raining seasons or after a heavy rainfall
C. The buildings and ground level (height) of adjoining properties (you don't want your neighbour's property pushing water towards yours.
D. The present road level and the possible road level in future (if not tarred yet) - normally your site would be below this level.
(That should do for now).
Getting ready to Build.
1. Design Stage:
It is advisable to get a professionals involved (an Architect and a Structural Engineer) at least; they would be responsible for visiting and producing a relevant drawings for 'your' use; This is not a dry land where you make away with some compromise; be sure to adhere strictly to the design and professional advice; in summary "Baba Joe" cannot be trusted even with his 100 years bricklaying experience.
2. Foundation Work:
Most times, there is no need to dig for foundation (as practiced on strong Laterite soils) - The road level in swampy / waterlogged site are usually (already) higher than the site - this is so because the community or sellers would have done a great deal of filling to make the road use-able or accessible even during raining season; you might even find your site level about 900mm or more below road level so why dig?
Clear the site of soft bodies (wood, leaves, dirts etc), this can be done with a rake - the point is we want a level ground; once that has been achieved, cast a light binding (after setting out the structure) - light binding could be as weak as 1:10 cement concrete mix; the idea is to allow our form-work sit properly on firm ground and not sink under weight, its also advisable to ensure the blinding is plum i.e level all round.
3. Picking Levels:
Already in our exercise during the raining season above; I advised that your know the water levels; the professionals on your job and indeed yourself should ensure you're at least 450mm - 600mm above this level - especially in areas where the road is un-tarred. What this also means is that you'll be importing a lot of filling sand in the nearest future into the site to meet up with the designed ground level and eventually drain into the road gutter levels.
4. Reinforcement works:
It is believed your structural engineer got this covered already, but giving you an insight to what to expect;
Beams = 16mm
Rings = 8mm
Columns = 12mm for bungalows and 16mm for storey
German / Ground Floor = Wire mesh or 8mm or 10mm
The nature of your soil and structure would determine if you'll be using more or less.
5. Soakaway / Septic tank:
On a waterlogged site there is usually no soak away because the water level is so high, surface water cannot be absorbed further in, except you have ground water gushing out. lol. so its septic tank we would be discussing. The major method is precasting and sinking while a more difficult method is casting in-place, you would however require big pumps, water cement and a lot of technical knowledge to succeed with this method; so it's advisable you precast and sink - be careful to avoid break or cracks during this process.
6. General advice:
a. As soon as possible fill 'your' site to road level, this would allow you work all season. If not, water around your building might limit what can be done during the raining season.
b. Your fence should be on a deep raft, not some shallow one, because you're working during the dry season, there is the tendency to assume the ground is firm - it might eventually collapse during the raining season and thats an axtra cost.
c. If your site is swampy or waterlogged whether in dry or raining season, then it's advisable you fill the compound to a level where you have a solid land to work on; this applies to those buying water land (land that can accomodate a canoe anyday or time) - Your first work would be to import tippers of filling sand to allow you access to work.
Summary: THE BEST TIME TO BUILD IS THE DRY SEASON - BUT STUDY YOUR SITE DURING THE RAINING SEASON & INVOLVE PROFESSIONALS.
Thanks and Goodluck.