The first step is written, with a specific deadline
Not because formality is an end in itself, but because it determines what you can ask for afterwards.
The demand states exactly what is owed, gives a reasonable period, and sets out what will follow if it is not met. It is sent traceably.
It also has a practical effect: a significant share of cases end here, because until then the conversation had been verbal and the other side treated it as non-binding.
The options
Performance. You insist the work is done. It makes sense where the other side is capable of performing and where they are not easily replaced.
Rescission. You end the contract and ask for what you gave back. It makes sense where performance clearly will not happen, or where the delay has drained the deal of its point.
Damages. You claim the loss you suffered.
These are not chosen by emotion. The choice depends on what actually helps you and what can be proved.
Contractual penalties
A penalty makes life easier, because the exact loss does not have to be proved. So it is worth having in the contract.
But not every penalty clause works as written. If it is excessive relative to the interest it protects, it can be reduced. And if the loss exceeds the penalty, the difference is sometimes claimed separately.
So the calculation is not simply days multiplied by a percentage.
Be careful about acceptance
Accepting the work without objection has consequences. Silence is in certain cases treated as acceptance, especially where the contract says so expressly.
If something is wrong, object in writing and immediately, even if you expect it to be sorted out amicably.
Before you start proceedings
It is checked whether there is anyone to collect from. A judgment against an empty company is expenditure without result. See debt collection.
What I will need
The contract and its annexes. Evidence of what you performed and paid. The correspondence, messages included. Anything documenting the delay and the loss it caused.
This page provides general information and does not constitute legal advice on any specific case. Rights and deadlines depend on the facts and the applicable law.