The aim of paper is the evaluation and analysis of the change of rainwater runoff for the proposed extension of the building and the assessment of the impact of the extension of the building on the drainage of flood waters. The annex is situated on the northwest side of the SO1 building and on the northeast side of the SO2 building. The extension extends the existing surface and connects the two existing objects to a single entity. The proposed extension is located in the Q20 floodplain area of the Lobezsky stream and also in the built-up area of Sokolov. From the analysis of the territory, we found that the Q20 floodplain is based on a theoretical assumption, since this area was historically the most flooded in Q5. By evaluating the data and calculating the total amount of rainwater, we obtain real data on the drainage conditions in the site. The calculation of rainwater also includes the design of the infiltration body. Furthermore, we analyze the impact of the extension of the building on the drainage conditions and the assessment of the outflow of flood waters. We evaluate the theoretical directions of runoff during floods or during rainstorms