The Villa Savoye might not be on most must-visit lists for Paris, but the building is one of the seminal works of modernist architecture, and just a half-hour train ride from central Paris. It's on the RER Ligne A; once you arrive in Poissy you can either take a bus to the villa or it's about a 20-minute walk. Here, from 1929 to 1931, Le Corbusier created a house that expressed his vision of a new architecture and especially the five points that were central to it. These were the replacement of load-bearing walls with supporting columns, the creation of roof gardens, a free floor plan, free facade, and horizontal windows.
The Villa Savoye is much more than a country house for a bourgeois Parisian family, it's a three-dimensional manifesto expressing Corbusier's new design program. There are free guided tours in English (check the website for details) which are especially helpful if you aren't an architect or traveling with one. There is literature available, however, to point out the highlights of this incredibly influential but understated and restrained masterpiece if you don't join a tour.