Hello Simon,
the file you uploaded is not a site plan, but in my opinion only a sketch or a mixture of a sketch and a digital plan.
Nevertheless, be careful, because such deviations do occur, but this problem is hardly explained here in the forum.
As already correctly mentioned above, you will need a surveyor anyway for your construction project. It would be advisable to consult this person immediately or contact the surveyor who recently carried out the division of the property. They can provide you with exact/reliable property data or digitally forward it to your architect/builder.
Regarding the issue of setback areas or distances between buildings, I prefer not to write anything because, based on your descriptions and the sketch, there are several "ifs" that need to be queried/handled. For this, more information is required.
If you are unsure or have questions, just call, my contact details are listed in my profile.
Best regards Dirk Grafe