Context : The way massive pre-main sequence ( pre-MS ) binaries form and evolve is by large an open question . Here we systematically address this topic from the perspective of stellar structure and evolution models . We are looking at the pre-MS detached evolution of massive binaries that evolve , through accretion growth , from a small mass stellar cores to massive zero age main sequence binaries . Aims : We explore the initial conditions that lead to detached binary evolution of massive pre-MS binaries and ask how large a fraction of the observed binary systems may have been initially formed as low-mass protobinaries and later undergone a significant accretion phase while remaining detached . Methods : We develop a family of analytic models to describe the orbital separation , a , and mass ratio , q , evolution . For a given mass accretion rate onto the binary system , we define a recipe for distributing this mass between the two components . For this we introduce a parameter \eta , such that \tfrac { \dot { M } _ { 2 } } { \dot { M } _ { 1 } } = q ^ { \eta } at any time , to determine the binary mass ratio evolution . Depending on the choice of \eta , any type of mass ratio evolution is possible . Furthermore , we use MESA , a detailed stellar structure code , to calculate an extensive grid of binary sequences where a protobinary undergoes accretion , and we identify the initial conditions that separates detached from non-detached pre-MS binary evolution . Results : A value of \eta around 2 allows accretion growth in detached systems to form close massive binaries on the Zero Age Main-Sequence with minimum orbital periods down to about 1.2 days for M _ { 1 ,ZAMS } = 20 - 30 M _ { \odot } twin-binaries . \eta = 2 can also reproduce the observed population of binary systems with primary stars above 6 M _ { \odot } . Conclusions : The whole observed range of massive close binaries can form via accretion growth in detached systems , making the binary formation channel of accretion growth a strong contender to explain the formation of massive close binaries , including progenitors of coalescing binary black holes .