Hi, Jo here, confessing that she forgot today was her blog day. Clearly I'm still not in sync with 2016!
So I've pulled out an old post that's still of interest, where we can enjoy an imaginary visit to a London town house.
I just picked up a book called The Classic English Town House, by Rachel Stewart. It has Adam's plan for Lord Derby's house at 26, Grosvenor Square. I've seen it before, but this time I decided to simplify it to make the layout easier to follow, so we could imagine visiting it.
It's a plan for the refurbishing of the interior of an existing house, probably a similar one to these. 
The plan is only of the ground floor and first floor, which is the second to an American. There were probably two more floors of bedrooms and servants' quarters in the attics. There was certainly a basement with storerooms, kitchens and other offices.
So here's the simplified plan of the ground floor. Remember, you can click on an image to see it in a larger size.
The house was about 30 feet wide.

Visiting the House.
(All that follows is my interpretation of what I've read. I'd love to have additional or conflicting information.)
Simple folk like us.
We knock at the door, which is in the center of the hall. Of course, being of the lower orders we're shooed away, or to the servents' entrance, but let's assume we have some reasonable reason for invading. We'll be left in the hall, or perhaps offered a chair under the beady eye of the porter. His quarters are the tiny square in the hall there.
If we're respectable enough, we would be taken to the anteroom whilst the footman goes to inform milord or milady of our arrival.
Dinner guests
Let's assume we're invited to the house to dine. You'll see that the dining room is on the ground floor. This is efficiently closer to the kitchens, but it's also on the public floor of the house, not the more intimate first floor. We will probably be taken through the anteroom to the parlour before the meal. If it is a gentlemen's meal, which they often were -- business lunches, in effect, and in the 18th century an afternoon meal -- then the diners might progress no further than the dining room.
However you'll see that beyond the dining room is the library, which would largely be a male domain, so some might be invited there after the meal for further discussions. Beyond that lies milord's dressing room. There would be a single bed there for his use when not visiting his wife on the upper floor. Interesting to see both here and in milady's suite a water closet, though it's not clear where it discharges to.
If the meal was for gentlemen and ladies then the ladies would leave and go to one of the drawing rooms on the first floor to enjoy tea or coffee while the gentlemen got down to serious business over port and such. 
A Rout
Now let's consider that popular form of entertaining, the rout. I assume, but do not know, that the name comes from route, because it was a matter of traveling through the house, seeing and being seen. For these events as much furniture as possible would be removed.
I've cut down to the drawing rooms. There's a reason for three. We enter the house and divest ourselves of cloaks and such, giving them into the care of our maid or valet. We go upstairs and work our way through the first, second, and third drawing rooms, where there's a convenient exit to the stairs and off we go!
A Ball?
These townhouses weren't suitable for grand balls, but the Third Drawing Room, at 22 x 33 feet could hold a small dance. In that case the other two rooms could be for cards or gossip.
Other Entertainments
There could be a musical evening, a card party, a salon of poetry and discussion, all in one or more drawing rooms. This museum room has no furniture, but we could enjoy the like.

I hope you enjoyed this visit. Remember, too, that this would be one of the grander townhouses in London, where land was scarce and expensive. Others were narrower and might have fewer floors.
The book The Classic English Town House points out that these small aristocratic houses only worked because they were surrounded by services. In the country the nobility could house many servants, incluing maintenance staff, services such as a bakery and brewhouse, and large storage areas. That wasn't possible in a London townhouse, but extra servants, food, drink, repair people and such were all around, for there wasn't much separation between rich and poor. If the house didn't have stables there would be a livery stable not far away. 
In The Viscount Needs a Wife, Kitty goes to inspect their London house, which has been unused for a while. It's quite like this one. She's already dealt, or sort of, with the country house you see in the background of the cover. She likes the town house more. The book will be out in April, but it can be ordered now.
I hope you enjoyed this.
Cheers
Jo