Viteszlav: I have added some info to a thread that addresses your questions — see link below. But briefly, yes the house is a private home. Unfortunately the owners have been bothered by people milling about in the past so I wouldn’t count on being let in to tour the place. However one can walk down the street and admire it.
I was in Versailles last week but I am so stupid! I thought that Pavillon Mars was house number 6, and, by the way, I could not find the number 4! Next to six was a low house and than a big one on the corner with the broad Boulevard...? So where is that Pavillon?
I went to the house last November as part of my travels. I can't remember the number, but it has a lovely smart sign in grey slate (?) over the door with the name, 'Pavillon Mars' so you might have missed it. The house has been wonderfully restored as it is a heritage building, and I had a peek inside. There weren't exactly crowds of people bothering the owners It was just a quiet street. Me and my friend went and looked round the grounds of Versailles as well, where Lucy got many of her ideas, and we even had a cup of tea, as we looked at the canal. A beautiful inspiring place!
There is no way to "peek inside" the Pavillon Mars. It has no open windows facing the street and the garden is hidden by a high wall. I met the owners and have been in touch with them since I first visited in 1997. According to them, people have indeed bothered them - mainly because of the house's historical connection to Mdlle. Mars and Napoleon. The house was not restored by the current owners. It was in good shape when they bought it. It has been left much as it was.
Some furnishings are original to the house as is a portrait of Mars. These would have been part of the estate when Lucile lived there. Lucile's touch can also still be seen here and there, such as in the recess lighting in the small library/sitting room —— this was part of the 1912-13 Elsie de Wolfe redecoration that Lucile commissioned. Also the upstairs bathroom still has the built-in dressing table Elsie installed for Lucile and there is a little dressing room with cabinets and closets galore, which matches the bathroom woodwork, indicating it was part of Lucile's decorating scheme. The garden is a gem of a place. The lilac trees/bushes Lucile planted are still there, and one can still make out where family photos were taken, judging by the trees, though they are a bit larger now! The latticed wall behind the Duff Gordons in the familiar garden photo of them is still recognizable, though shrubs have grown up to partially obscure it.
I don't know when you last visited, but it had certainly been recently renovated, and re-painted by someone! The paint was barely dry when I saw it, as posted above. You can see through the front door and some of the windows that face the street. It has a very new looking sign over the door, which is grey and engraved with the name. It has been beautifully done, and looks really pretty. It is certainly worth a look if anyone is in the area.