How high are default interest rates?

Exactly: not 12% p.a., but 1% per each started new month.

The German tax office takes 0.5% per month. ;) But I don’t quite understand how this can be applied to construction. I also seem to recall that even the tax office currently has an ongoing review to determine whether the interest is justified/up-to-date.

The tax office in D charges, I believe, 6% p.a., I just saw that on Google. Not bad, Mr. Specht, I say.
Maybe I'll try the 6%. Whether that corresponds to Belgian law, I don't know. I'll just assume that the differences aren't that huge.

You can write whatever you want on paper, that wouldn't interest me at all as an entrepreneur. 6-12% is also not market compliant. Tax office = state, they have the upper hand.

What do you get on the market for overnight money? 0.0%? Or rather negative interest? So I would say you have to pay the construction contractor -0.3% negative interest.

What do you get on the market for overnight money? 0.0%? Or rather negative interest rates? Well, I would say you have to pay the contractor -0.3% negative interest

It's not that bad yet. The base interest rate for 2020 is -0.88%, and §288 of the Building Code sets the default interest rate 5 percentage points above that. That would be 4.12% (and calculated exactly by day, also in leap years for the actual time and assuming a non-leap year, as well as without compound interest). That means when December has passed, November still has its 30,000 days and does not suddenly have 31,236 days effectively because of compound interest ;-)

I also seem to remember that even at the tax office there is currently an audit underway to determine whether the interest rate is justified/up-to-date.

Indeed, this issue has been pending for some time at the Federal Constitutional Court for a decision. However, it has already leaked that the Federal Constitutional Court apparently views the state's interest rate as a kind of overdraft interest rate and therefore considers the 6% per year to be justified, as this is still quite favorable for an overdraft interest rate.

Since the written reminder was already sent in September, I can now also charge default interest.

Interesting thread. The question, however, is on which amount you want to calculate default interest? And from when? For what reason?

Forget it, the defects do not have the value that default interest could have any effect.