The extendible building post includes a telescoping hydraulic cylinder integrated within a construction member externally configured and dimensioned essentially identically to a conventional building construction member, e.g., a four-by-four or six-by-six length of lumber, or alternatively a plastic column. The hydraulic system may be formed of stock plastic pipe and fittings, and may use water as the working fluid. The device may include insulation and a heating system to prevent freezing when used in such an environment, with certain types of insulation also providing additional rigidity for the hydraulic assembly.