Can we plan a basement/cellar for a southwestern-facing house?
According to Vastu Shastra, there are better ideas than building a cellar or basement in a house facing southwest. Putting a basement toward the southwest could be wrong because that orientation is linked to stability, wealth, and longevity.
Also, the basement's susceptibility to moisture and humidity can reduce the house's energy balance. To better use the extra room, you should look at Vastu-compliant alternatives.