Interior plaster, craftsman suddenly wants more money

First, the company made me an offer with which I did not agree, amount X per sqm per cm, then we verbally agreed on the amount X per sqm.

Mathematically, one would probably have to conclude here that X = X, meaning the offered price was accepted. However, if the verbal agreement means Y, then continue reading.

A contract for work regarding Y has been concluded. I assume that there was no written acceptance of X either.

Now the plastering is almost finished and the craftsman suddenly wants the amount X per sqm and per cm after all, is that lawful?

No. A contract amendment requires an agreement between you. However, I would be interested in the justification for the expected higher price.

Furthermore, the offer stated that the walls would be plastered smoothly but without specifying Q3 or Q4, unfortunately, the plaster is not as smooth in many places as in others, so I have to do rework before painting.

Did you discuss during the contract negotiations that you want to paint the walls afterwards without wallpapering or your own reworking? Was it clear enough that this could have become part of the contract?

The garage driveway is completely dirty with plaster because the work was done untidily.

Those good people will have to clean that up at their own expense. No question.

In addition, the plasterer scratched some of the windows on the inside reveals.

That should also probably be fixed by the plasterers. What surprises me here is that normally the windows are protected for plastering with adhesive film.

I told the company to stop and said I would only pay once the damages were fixed and everything was cleaned.

So you terminated the contract and "chased the people off the premises." However, I cannot see here that you gave prior notice of proper performance and an opportunity for defect rectification. Also, not in a verifiable written form. Therefore, you did not have a right of termination. From here on it gets complicated, because you cannot just withhold payment like that.

I will only pay what we have agreed so far and will have an expert come for the damages.

First, of course, you have to report the defects in writing. The contractors have a right to attempt to remedy the defects. Essentially, under contract law for work, there is a claim for payment only if the agreed-upon result has been provided without significant defects, unless otherwise agreed. Here, however, you have terminated the contract and unlawfully prevented the contractors from performing their work.

An expert is ALWAYS a good idea. I also do not understand why this person is not accompanying your construction project anyway. A lawyer is also a good idea, but I see great potential for agreement here, as the good people definitely want to continue.

What I noticed overall and do not like here as much:

1. No written contract = no concrete agreements on the scope of services and payment. Regarding the price, it is statement against statement. It would be questionable whether "your" price expectation (i.e., Y) is below the usual level or within the normal range. Then X would have to be too expensive compared to the generally valid prices. This also offers potential for agreement regarding the difference.

2. Were the films on the windows removed by you? Or were they not there at all?

3. Termination of the contract and sending away the contractors without termination rights and without prior legal consideration.

Now a short bit of cleverness (almost usual):

For smooth construction execution, one should inform oneself about the reputation of the executing companies before signing the contract. All services have their price. The search for bargains then leads to companies that often do not perform well. It is in the nature of things that one offers one’s services at the general market price and does not (voluntarily) contract with bargain hunters. If these companies accept such orders, then lucrative ones are usually not to be had. One wonders why that is.

My course of action would be:

1. Try to reach an amicable agreement to complete the work and remedy defects. This requires willingness to compromise on both sides.

If 1. cannot be reached:

2. Consult a lawyer f (specialist lawyer for construction law) and coordinate further procedure. Possibly this will also facilitate step 1. A lawyer’s letter often works wonders.

3. Commission an expert if the lawyer also recommends it.

Good luck.
V.