  We present a stochastic approach to the spatial clustering of dark matter
haloes in Lagrangian space. Our formalism is based on a local formulation of
the `excursion set' approach by Bond et al., which automatically accounts for
the `cloud-in-cloud' problem in the identification of bound systems. Our method
allows to calculate correlation functions of haloes in Lagrangian space using
either a multi-dimensional Fokker-Planck equation with suitable boundary
conditions or an array of Langevin equations with spatially correlated random
forces. We compare the results of our method with theoretical predictions for
the halo auto-correlation function considered in the literature and find good
agreement with the results recently obtained within a treatment of halo
clustering in terms of `counting fields' by Catelan et al.. The possible effect
of spatial correlations on numerical simulations of halo merger trees is
finally discussed.
