Of course, Polish law and Polish warranty apply
That naturally depends (e.g., on who offers or requests which service where or where the contract is concluded). Moreover, special provisions can also be agreed upon in the contract. In doing so, one should of course also consider where the differences and advantages/disadvantages lie in the respective warranty rights. The core problem, however, was the question: How do I ensure that - what is ordered is what is desired - what is delivered is what was ordered (and not whether what was delivered is basically okay and tip-top! A difference ). Our fence is also from Poland, but we ordered from a German (practically a neighbor) who planned and is above all also our contractual partner. He is so to speak the link between the German customer and the Polish craftsman. He took the measurements and before placing the order we had a CAD drawing of the fence on the table. References were available as photos on a CD or by personal inspection in the neighborhood (with fences naturally easier than with stairs). The Poles then only come for installation. This means: Either you select the product from a catalog (for furniture and stairs you need a sample catalog that shows what the surface looks like over a large area) or you have a model (production drawing, computer animation, 3-D model, whatever) created before placing the order. Or you visit an exhibition, but the questioner already knows that. Once all questions are resolved, you can place the order.