The construction of the BEaTriX (Beam Expander Testing X-ray) facility is ongoing at INAF/Osservatorio astronomico di Brera. The facility will generate a broad (170 x 60 mm2 ), uniform and low-divergent (1.5 arcsec HEW) X-ray beam within a small lab (∼ 9 x 18 m2 ), using an X-ray microfocus source, a paraboloidal mirror, a monochromation system based on a combination of symmetrically cut and asymmetrically-cut crystals in Bragg diffraction configuration. Once completed, BEaTriX can be used to test the Silicon Pore Optics modules of the ATHENA X-ray observatory, as well as other optics, like the ones of the Arcus mission. The facility is designed to operate at 1.49 keV and 4.51 keV, by using two fixed beam lines, equipped with the necessary optical elements. The first beam line to be completed will be at 4.51 keV and will prove the BEaTriX concept. Silicon crystals are used at this energy and four symmetric diffractions, with appropriate tilt of some crystals, will provide the spectral filtering at the required level to return the desired divergence. Owing to the quite short range necessary to obtain a parallel beam with this setup, a low vacuum level (10-3 mbar) can be used without a significant beam extinction. In addition to a modular vacuum approach, the low vacuum will allow us to reduce the time required to evacuate the tank, thus enabling to demonstrate a test rate that will match the ATHENA SPO production of 3 MM/day. In this paper, we report the design of the facility and the construction progress.