The soil survey is the cheapest line item, and it sets the structural budget
A geotechnical report costs a fraction of a per cent of the project. Two lines inside it decide the foundation, the excavation and the seismic design — and skimping only shows once the concrete is down.
The geotechnical survey is close to the cheapest thing you will buy on a building project, which is why it so often goes to whoever quotes lowest. It is also the document that sets your foundation type, your excavation method and the spectrum the structure is designed against for earthquake load. A line item worth a fraction of a per cent decides something closer to a tenth of the shell cost.
Start with how many boreholes were drilled, and where. Open the site plan attached to the report and check where the drilling points fall: we regularly see reports built on boreholes sunk entirely outside the building footprint. Soil can change within ten metres on the same plot, particularly on ground that was once a stream bed and has since been filled. Text adapted from a neighbouring parcel's report is common as well — the drilling dates and coordinates usually give it away.
The second line that matters is the site class. Turkey's seismic code grades ground from ZA down to ZF, and that grade directly changes the design spectrum used to calculate the earthquake load carried by the structure above. Graded better than it is, the building gets designed weaker than it should be; graded worse, you pay for reinforcement and section sizes nobody needed. The report should state plainly whether the class came from a shear-wave velocity measurement or from SPT blow counts alone.
Third is the liquefaction assessment. In saturated, loose sandy soils there is a real possibility of the ground losing its bearing capacity during an earthquake, and that has to be calculated separately. Along the Istanbul shoreline and in alluvium-filled valleys it cannot be skipped. Where liquefaction risk is found, the foundation type changes and ground improvement becomes necessary — both of which belong in the budget before the permit, not after it.
Fourth is the groundwater level. That single line determines the type of basement waterproofing, and whether the excavation needs shoring and dewatering. If the water table sits above the excavation level you will be pumping continuously, and it is the cost most often left out of the programme. The season the measurement was taken in matters too: a level recorded in late summer is not the level you will meet in spring.
If the report comes back as ZF, or calls for a site-specific investigation, the schedule has to be rebuilt around it. That is not the bad outcome. The bad outcome is the same ground being waved through as a standard class, and the problem surfacing at foundation stage.
The most useful half hour an owner can spend is this: get hold of the report, open the borehole logs and the site plan, and see for yourself whether the drilling points fall inside the building footprint. Then check the date. A survey commissioned five years ago for a different scheme cannot be reused for today's building without being reassessed. Skip that check and the gap it hides cannot be corrected once the foundation concrete is poured.