A plant for building tyres comprises a plurality of forming drums (14) and at least one pair of removably associated support rings (15), a station (16) for coupling pairs of support rings (15) to the respective forming drum (14), a first sequence of building stations (18a-18g) for tyres being processed, each operating on a forming drum (14) associated with one of the pairs of support rings (15), a removing station (19) for removing pairs of support rings (15) from the forming drum (14), a second sequence of building stations (21 a-21 c) of tyres being processed, each operating on a forming drum (14) detached from the pair of support rings (15), a shaping station (22) in which a shaped tyre being processed is separated from the forming drum (14) detached from the pair of support rings (15), a forming drums temporary storage (24) for placing in standby the forming drums (14) separated from the tyre being processed coming from the shaping station (22).