Is the financing plan realistic?

And what exactly is included in the ancillary construction costs of 40,000?
I calculate all the connection costs in my calculation as part of the land.

According to the house offer, all the groundwork including the foundation slab (listed with ~19,000) as well as the entire interior of the house (bathroom and sanitary facilities, floors, painting work, electrical work,... including the materials for all of that) are included. All of this is covered by the 272,000 according to the offer.

So what else could still arise, so that I have an additional 40,000 in ancillary construction costs on top of the offer?

Read the pinned post on the topic of [Baunebenkosten] in the Baukosten forum. Earthworks are usually only included up to 30cm. Rainwater drainage not at all, etc.
Take a look: I invite you to browse the forum

Regards