## 1 Introduction  
By concluding a contract of works, the contractor undertakes to hand over the facility provided for in the contract and the contracting authority to pay a specified remuneration. The key stage in mutual relations is the acceptance of works. However, the obligations of the parties do not end with the acceptance. Under legal regulations, as well as the usual relevant contractual provisions, the contractor takes responsibility for the defectiveness and noncompliance of the performed works with the contract. In the Polish legal environment, the rules of liability are determined by guarantee and warranty. These two types of liability of the contractor operate independently of each other, showing both similarities and significant differences. The main difference is that granting the guarantee is a voluntary declaration by the contractor to assume responsibility for defects in the works performed, which is regulated in the contract. On the other hand, the warranty is legally binding and is regulated in detail in the provisions of the Civil Code [1]. According to Article 556 of the Civil Code, the warranty is an inalienable right of the buyer, while the seller is liable to the buyer if the item sold has a physical or legal defect. A physical defect is defined as one of four possible situations concerning the sold item:  
1. There are no features that such a thing should have in view of the purpose in the contract, as indicated or resulting from the circumstances or purpose. There are no features that the seller has provided the buyer with, including the presentation of a sample or pattern. Not fit for the purpose of which the buyer informed the seller at the conclusion of the contract, and the seller did not object to such a purpose. The item is issued to the buyer in an incomplete state. However, when identifying potential defects, it is important to keep in mind the limited time for reporting them. In the case of a property defect, it is a period of five years from the date of handing it over to the buyer, which, with regard to construction works, takes place at the time of acceptance of the works [2,3]. It should be noted that the detailed specifications of the contractor's liability are described in the national legislation of individual countries. While the paper refers to Polish regulations, the laws of other countries also include the concept of warranty, differing in only some formal and procedural details. In the context of the warranty, the provisions of the Civil Code do not describe the fault but the defect of the item sold. In this paper, the notions of fault and defect are treated as equivalent due to the way the issue is described in Polish publications, where these terms are also interchangeable. Defects are a common phenomenon in the construction industry in all countries. Contractors, as well as investors, should pay special attention to them as they can have a significant impact on the costs and required resources of the project. In the literature, there are many publications related to the frequency of defects in residential buildings conducted in various countries [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. However, the vast majority of the publications are related to defects reported during acceptance. The research of the defects in buildings occurring during the warranty period may become an important contribution to the analysis of defects. The aim of the article is to present the results of preliminary research of the defects in residential buildings occurring during the warranty period. Due to the small amount of data, the research results cannot be generalized but allow for the formulation of research hypotheses that will be verified in future studies. This paper is organized as follows. Section 2 contains the literature review. Section 3 introduces formal and legal conditions relating to defects reported during the warranty period. Section 4 describes the analysis method and results. Finally, Section 5 presents conclusions.