Basement planning

Hello forum, we are planning an extension to our existing house (built in ’76). It will be a double garage (outside 9*7m) with residential space on top and half of it will be a roof terrace. Now I am thinking about a cellar. The existing house has a masonry cellar, and according to my father-in-law, the cellar is flooded in winter (not in summer), which used to cause problems because the masonry is only coated on the outside with a bitumen layer and on the inside with waterproof plaster. The new cellar would have to be attached to the old one, and there would also need to be an access from the old cellar. Now I have many questions, as I am looking for ideas for the project and have not yet spoken to the architect about it. How much does a cellar roughly cost (I know, you can’t really say precisely) as a black or white tank? Is it even feasible with a black tank under these conditions? I would do a black tank myself; we can handle the masonry work here... how do costs look like with self-work? Also, the question is how the sealing between the two cellars works, i.e., at the passage; are there reliable systems for sealing? I look forward to your answers, suggestions, critiques, and ideas. Friendly greetings from Bocholt Peter

So in the case of previous water damage in the old cellar, I wouldn’t break open the painstakingly sealed system again. Do you have a sufficiently large plot of land? Then place a soil mound with an earth cellar right in the middle. It’s great for planting kitchen and medicinal herbs and the best pantry you could wish for to store potatoes, etc.