Slovenia has for the last two decades a positive trend in constructing and renovating of small hydro power plants. Renovated ones are often those left behind by industries that shot down lately. Governmental implementation of support scheme was a reason that many of them had been eligible for the last years, even those with poor optimization and efficiency. This scheme stopped in 2015 and was substituted with a new one based on an auction. A new method of support brings large uncertainty between design engineers. Multiplying this with uncertainties of hydrological conditions makes investment in a small hydro power plant like Osolin a risky investment. To avoid potentially poor solutions, there is a necessity of making pre-investment studies. In pre-investment studies we introduce and check different solutions. If the solutions meet obligated requirements, the last step is to propose the best one.