This topic is dependant on three things: What is being built, what options are available, and the condition of the existing concrete.
If you are unsure what concrete grade you need, click here for more info.
If you’re building a house extension that needs piling, and concrete is obstructing the process, the concrete needs to be removed/excavated in the areas where piles are driven. From there, you can back-fill around the piles once they’ve reached their set depth, saving time and money when it’s time for concrete installation.
However, if you’re excavating for pouring a slab and find an existing one, you CAN build on top of this providing you have confirmed the following: the condition of the found existing concrete is good (no cracks or deterioration) as well as knowing how thick the layer of concrete is.
That being said, you CANNOT pour directly on top of an existing concrete slab. The reason is that if there are any cracks, they will eventually transfer to the new slab, as there’s no buffer between them. Instead, you can use MOT or a layer of sand/stone 75mm thick to prevent these potential issues.
Failing these methods, and if the condition of the existing concrete cannot be verified, it is advised to remove it. If it does not exist on the technical drawings, it is best to be safe than sorry. You can remove concrete easily with a pecker drill, or a handheld hammer drill if there is no digger on site.
To conclude, you absolutely can build on top of an existing concrete slab, as long as the above steps are taken.