In building tyres, an intermediate portion of a carcass structure is built at a time on a respective forming drum within an intermediate forming line. The forming drum with the intermediate portion of the carcass structure is made available at a first transfer location. The intermediate portion of the carcass structure is completed through n completion locations with n≥1 and then transferred for the subsequent assembling with a crown structure to a second transfer location. At an operating moment in which the first transfer location is occupied by a forming drum coming from the intermediate forming line or intended for the intermediate forming line, from one to n locations selected from the n completion locations and the second transfer location are occupied by respective forming drums.