Have existing buildings thoroughly inspected before renovation

Hello! We have begun planning the renovation of our 1970s hipped roof bungalow. For brainstorming ideas, we consulted several companies. They expressed differing opinions about the condition of our building, so as a precaution we would like to consult an independent expert. Topics naturally include energy-efficient measures, but foremost the absolute foundation: is our building safe, can it withstand a renovation or an additional storey, and our problem child: a flat roof extension from 2013 is reportedly sagging according to an engineer. He advises shoring it up. We bought the building last year. We now want to know how we can protect our "baby" from further damage and determine the actual current condition in order to know how to proceed. The companies want to sell, and we are not sure if they recognize the risks. Ultimately, we want more living space for the children, but it should be safe and durable. One more piece of information: we live in the moor. To our knowledge, the building – and also significantly older buildings in the immediate neighborhood – were built without foundation piles. This is said to be because of a kind of sand hill. The old main building is brick-faced and shows no cracks or damage in its original condition. The building is partially basemented. Back to the question: so who can advise us competently and be our "specialist" for this matter? Greetings from the Oldenburg area