The way a bit 'longer but easier is as follows:
arriving in Siena continue towards Grosseto, and after about 10 km on the highway to Grosseto take the exit of "San Rocco a Pilli", cross the village and continue on the old road to Siena. About 2/3 km after San Rocco a Pilli, you find the fraction of Sant'Andrea in Montecchio. The entrance to the farmhouse can be found on your right marked by a large stone with the words "Agriturismo di Montecchino" (see entrance from the street). Enter the dirt road and follow the "Reception" sign.
GPS coordinates: Latitude: 43.280552 - Longitude: 11.298485
From Siena To reach the farm from the end of Piazza Gramsci take the urban line n. s30 (PIAZZA GRAMSCI - SAN ROCCO - CASTELLO) direction San Rocco a Pilli, get off at the second stop of Sant'Andrea in Montecchio.
The Siena railway station is located in piazza Carlo Rosselli. From the train station take the bus to Piazza Gramsci, then change buses and get on s30 (PIAZZA GRAMSCI - SAN ROCCO - CASTELLO) direction San Rocco a Pilli, get off at the second stop of Sant'Andrea in Montecchio.