This must be done necessarily and strictly. For geological survey of the site, contract a good experienced local organization. Usually they will also do the geodesy of the site.
For design work, hire designers who specialize in working in seismic areas. Designers working in non-seismic environments may calculate and design a house incorrectly. This does not mean that they are bad specialists, it means that seismic has a lot of specific technical solutions, but they do not have such experience.
Don’t listen to the talk “why do you need to spend money, we were doing geology nearby (we’ve been building here for a long time and everyone is happy), let’s take the data by analogy.” Do the geology of your particular site, and make at least two holes exactly in the spot where the building will be located. The technical specifications for geology and geodesy are made by designers, so by the time of the survey you need to decide on the designers. They will also decide on the approximate area and location of the building.
Keep in mind that in old houses there were almost always cellars either under the house or nearby. Find out. If there is (was) a cellar, and it falls under a building site, or is very close, do not fill it up. It may be necessary to dig it up. The designer must receive this information and then he will decide what to do with it, how to strengthen the foundation, or what measures to take.
Keep in mind that 99% of private houses near the sea do not have a central sewer system. The so-called septic tanks (the marketing euphonious name for a cesspool). Old houses 100% have this cesspool. It is imperative to find out where it is located, and the surveyor must accurately mark it on the plan. This is very important information for the designer, because if the hole falls into the building area of a new house, or closer than 4…6 meters, then the designer will definitely take this into account. The fact is that a cesspool is a place that is constantly soaked with moisture. With wide and deep ground weakening. (I’m sure you’ve seen that the cesspool area settles over time relative to the surrounding soil, and this is without load, from its own weight!). That is, if measures are not taken to strengthen the foundation, the foundation of the new building will certainly sag in this place. With the ensuing very high costs of restoring the strength of the building. Often restoration is impossible or does not make financial sense.