In short: what happens and what is up to you
The owner does not pick the foundation type; the engineer sets it based on the geotechnical study. The client's job at this stage is different: do not move the house on the plot, decide early where service entries will pass through the foundations, and get photos and records of everything that will be buried within a few weeks.
On site you will see an excavation or trenches, reinforcement cages and concrete being poured. It looks rough and quick. But this is where mistakes are made that later cannot be seen or cheaply put right.
What decides the foundation type
Pad footings under columns, strip footings under walls, a raft, piles: the choice comes from calculations based on the actual bearing capacity of the ground on your plot. The neighbour had a raft and their house is fine is not an argument. Two plots across a road can be very different.
Take a house in Castelldefels. Near the sea the ground can be sandy with a higher water table; on the Garraf slopes you may find rock next to made ground. In the first case the concern is settlement and water; in the second, how to bear the house on mixed ground. The difference between a topographic survey and a geotechnical study is covered in its own article; here it is enough to know that foundations are not designed without a geotechnical report.
The sequence on site
First comes setting out: the surveyor marks the house grid on the plot. Then excavation for footings or raft, a blinding layer so the steel does not sit on soil, and placing reinforcement and formwork. Before pouring, the site's technical supervision team checks the steel: bar sizes, spacing, cover and the starter bars for the columns. Only then is concrete ordered.
If you visit the site, go on the day the reinforcement is inspected. It is the last moment you can see what ends up inside.
What to build in now rather than later
Water, power and telecoms supplies, drainage, and pipes to the pool and garden all pass through or under the foundations. Sleeves are set for them in the right places and at the right depth in advance. Drilling finished concrete foundations is not always possible, and where it is, it is expensive and needs the engineer's approval.
The earthing electrode goes into the foundations too. If there is a basement or a buried plant room, waterproofing of the retaining walls and perimeter drainage are settled now. On a house with a pool and solar panels (we built one such private house through the full cycle in 2022 to 2024), the routes to the pool and plant area are fixed before the pour; afterwards there is simply no way through.
Concrete needs its time
Concrete gains strength gradually, and for the first days after pouring it is cured: in hot weather it is watered or covered so the surface does not dry out and crack, in rain the fresh pour is protected. When the foundations can take the next structure is decided by the technical supervision team, not by the schedule.
If a contractor offers to save a week at this stage, ask who will sign off that decision. A week saved is not worth cracks in the frame.
When the ground is not what the report said
A geotechnical study relies on a handful of boreholes, while the excavation opens up the whole plot. If digging finds an old foundation, a pocket of soft ground, water or rock where none was expected, the engineer decides: deepen the footings, replace the soil or change the foundation type. That changes the quantities and the price.
Agree in advance how such changes are handled: who records the finding, how quickly you receive a costing and who approves it. Then a surprise on site becomes a clear decision rather than an argument.
Which records to collect
Ask the contractor for photos of the reinforcement before the pour, with a tape measure in shot, the concrete delivery notes showing mix and arrival time, test results for any samples taken, and the site supervision notes for this stage. All of it will be useful at handover and, years later, if a question about a crack comes up.
The foundations are buried for good. The photo record is the only way to see how they were built.