The easiest way to do this quickly, even if your building is quite complex in shape, is to use a Color key.

The first thing I do is move the GRID sheet to just below the SYMBOLS FLAT sheet in the sheet list. This places the grid underneath the wall.

Then I add a Color key sheet effect to the GRID sheet and move it to the top of the list of sheet effects.

Ok all of that and make sure you have Solid fill and colour 6 selected as active. 6 is magenta - the same colour as was showing in the Color key effect.

Then draw a polygon on the GRID sheet that covers all the area where you don't want the grid to be visible. Here I have left a deliberate gap on the right hand side so that you can see how it was drawn.

Refresh, and the magenta polygon vanishes, taking the grid with it.

Once I move the nodes on that magenta polygon so that they cover that gap I left, I have a perfectly cropped grid.

Here is the file for that example

The other way I might do it, if I didn't want to be bothered with drawing a large and complicated magenta polygon, is by trimming the grid. Use the property picker tool to pick the grid so that you have all the right settings, and then hide everything but the floor and the grid. Then explode the grid and use Trim to entity from the Edit menu to trim the individual lines of the exploded grid to the inside of the floor extent. This usually takes longer to do than using a Color key, so I only rarely use it. There are also a few little glitches that can occur, such as lines that refuse to trim to the floor shape seen below. You can tidy them up by hand though using other types of trim.

This is the tidied up result. To make sure you don't keep accidentally selecting the separated lines of the grid, group them (as they were before you exploded them) and then make sure the grid is on the HEX/SQUARE GRID layer and freeze the layer.

As you can see, the result is the same. Perhaps the main disadvantage of trimming the grid is that it's not so easy to edit the shape of the floor.

This is the trimmed grid example.

I think it looks fine as it is, but you may be right about the docks. It would be a pity to build a city divided into 3 sections like that by water, and have no dock in sight.

Looks good to me :)

Looks good, but you've got a touch of transparency acne on those topmost wall sections. You can cure that by copying them onto a new sheet directly below and changing the fill of the copies to solid and any colour that isn't in that fill. 227 is a good one.

Asian Town

If you try to use the House tool to draw a long thin building it crashes every time.

Here is an FCW showing at what point the crash happens. (I saved after each successively longer building and attached the file once it crashed).

The limit seems to be somewhere around 120 feet long.

The reason such very long buildings are quite likely to be drawn is that the Mapping Guide indicates using them to draw the city walls. I noticed that the example map for that style has a building that is 175 feet long. I was trying to separate it out from the rest of the wall to take a screen shot of it with a result of the Distance tool to show you, when moving it caused a crash. Maybe it is something that has happened since the publication of Asian Town in November 2018?