The building used as the Timber Falls Motel is, in reality, the office for the Malibou Lake Mountain Club, a fairly ritzy (and private) estate and country club serving the posh neighborhood surrounding the lake.
It's hard to say whether the real estate around the lake was as sought after during the time of filming, as some of the houses and buildings used in the series were pretty dilapidated at the time, and not so "high end". At any rate, the club strictly forbids any photo taking of it or the homes around it. I contacted them months before my trip and asked if I might be allowed to walk around the neighborhood and take some photos of the Twin Peaks locations. I assumed that given the geeky nature of my request, they'd see there was no threat and allow me to wander around snapping pics. To make a long story short, they were not having any of it. They denied me any official access. Luckily, the road is public and I was able to sneak the necessary photos as we circled endlessly around the lake. As you can see from my photos, the lovely tree shading the motel and parking lot had fallen or been cut down since the filming of the show.
Interiors of the Timber Falls Motel were... you guessed it, filmed on sets.
It seems that they built and installed a facade of motel doors and windows over the front side of the garage/shed, next to the office. You'll notice that in the show, the doors are never opened from the outside. they cut to the set as Philip Gerard's door is kicked in. Check out the cheesy backdrop photo of trees behind Harry and Cooper!