The study addresses the assessment of additions planning to prevent overruns and delays in small business multi-family housing by using a probability matrix. Therefore, this article aims to provide statistical information on the additional costs encountered most frequently. The introduction highlights the frequency and negative impact of overruns on construction projects, especially in small businesses where resources are limited, and delays can be devastating. The main problem is the lack of effective tools to anticipate and manage these additional costs. The proposal is to implement a probability matrix that evaluates and categorizes possible additions according to their likelihood of occurrence and their impact on costs and times. The results show that, according to the sector evaluated, 0.166% of “Additions generated due to administrative costs” is the one with the greatest budgetary impact, representing S/. 40265.56 to be taken over by the contractor. The present study seeks to ensure that small firms treat these additional risks in order to anticipate and mitigate problems before they significantly affect the project.