Selling a house with unauthorized extensions – What to do?

My parents each own half of the house. Now the sale is pending and we children want to commission an appraisal. However, about 20 years ago my father made various structural changes without approval. My mother had no knowledge of this and there have been no problems so far. Now the question arises: How should we proceed to avoid subsequent fines? What consequences must we expect under these circumstances? In addition, my father is suffering from progressive Alzheimer’s and is to be placed in a nursing home. We would be grateful for helpful answers; we currently have no advice.

Now the question arises: How should we proceed to avoid retrospective fines? What consequences do we have to expect under these circumstances?

Get approval afterwards if possible. Otherwise, dismantle - that can certainly happen. The expert will not attribute much value to these illegal structures anyway if there is no approval.

That would then also affect a potential buyer. He will not pay the price that approved extensions would fetch. At the latest, his bank will pull the plug. Or the sale price must be low enough.

What kind of extensions are we talking about specifically?

After 20 years, no one cares anymore. Neither will the expert inquire about what is approved, nor the financing bank. If necessary, just don't provide a site plan without the extensions.

The expert will not ask what is approved

Then he is not an expert. He must (not want to, like to, or similar) compare the actual state with the approved state.
What do you think, how does an expert work? At the latest, if he takes the plans as a minimal work effort to calculate areas, he will notice, for example, that an extension is missing, which he has visited. And if he does not consider this, he is at best a trained butcher, but not an expert.
And if you don’t have idiots sitting at the bank, it will be noticed there as well. The banker enters 120 sqm into his appraisal tool (or also the broker), because that is what is stated in the living space calculation (or visible from the plans). Of course, the lending value may still fit because there is enough equity. But if it is a normal financing, the client will eventually wonder why the property was depreciated so much in his inquiry. The banker then says “it’s only 120 sqm of living space” and the client replies “but I measured 180, because 180 were also built.” That’s when the purchase falls through.

If necessary, simply don’t hand over a site plan without the extensions.

If necessary, refuse access to the building, do not release construction documents and do not release any cadastral maps or similar (which are accessible online in most federal states by experts and private persons). Ask the expert, in this special case, to simply write the price you mentioned or alternatively to roll dice.
You may have ideas that are detached from practical experience...

I bought a house 2 years ago, where the property office recently found that the garage is not on the site plan. The house was built in 2006, the garage was approved but was only built 2 years later. After the road renovation, all properties were newly surveyed for my residential house, because nothing is correct in the site plan from the "Middle Ages." Since the extensions are already 20 years old here, the house is probably even older.
Only a simple valuation report is made for a normal sale, and with regular financing, the bank only looks at the house from the outside.
If the extensions are not half on the neighboring property, nobody cares after 20 years.

I bought a house 2 years ago, where recently the land registry office found that the garage is not on the site plan.

And? What does something like that have to do with an appraisal or financing? You haven’t done anything wrong.

After the road renovation, all properties of my residential house were remeasured, because nothing is correct in the site plan from the "Middle Ages."

What do you mean by that?

It’s just a simple valuation report made for a normal sale.

How exactly is the appraisal then carried out? I would be interested to know how such an appraiser works.

For normal financings, the bank only looks at the house from the outside.

OK. And by just looking from the outside, the value is determined? Or how does "the bank" do that? I would also be interested, since that is kind of my job.