Creating the shapes
Ok, now that we have everything on the right place lets get started.
First were going to determine the size of the building. Create the two lines and connect them as shown in the image 1. Color black. Make sure the middle line, where the two lines meet, consists of three pixels and not two like the rest of the line. We do this because a rectangle looks smoother that way. Do not do this when the rectangle is in a pattern, because then it doesnt look smooth anymore.
Its time for the roof. We want the building to have two stories. Copy the lines we made earlier and use the move tool. While moving the line upward, hold shift so the object wont move to the side.
Finishing the outline
Copy the top lines, the flip horizontally and vertically. Move it into place. Connect the roof with the bottom two lines by adding two vertical lines of one pixel width on the outside. Do the same for the middle, but choose a light grey color ( 232, 232, 232). Now we have the outline of a rectangle. We want to use the light grey color for all the inner outlines. Remember this because its very important, we can apply this to all the rectangular shapes. Copy one of the two sets of lines we made earlier.
Name the layer bricks. Make it invisible or move it away for now. Well get back to it later. Make the original layer light grey ( 232, 232, 232). To save time, select that layer and fill it. See what I told you earlier about backgrounds? The innerlines are hardly noticable now. But well stick with this for now.
Filling in the outline
Lets start to fill in the outline. A good idea is to copy the outline now, because we might want to make changes on it later. Put the copied layer on invisible. Select the original layer and grab the Paint Bucket. The sun will be somewhere on the right side, so choose an average grey ( 156, 156, 156) and fill in the right area. Use a darker value for the left side, because thats the shady side. I used RGB 111,111,111. The roof color must be a bit lighter than the right side. Now we have the average isometric shape.
The building looks a bit boring as it is now, so lets expand it. The roof looks too flat, so lets make an inlet on top of it. Repeat Image 1, but cut of four pixels or two pieces at the ends. The outline we copied earlier comes in handy now, because we can use part of it for this. Place it directly on top of the light innerline, or the edge of the roof. Move it two pixels upward. Copy it and flip it vertically and horizontally. Copy the layer again. select one of them and fill in with the light grey: 232, 232, 232. Move the light grey one the same way as the first lines, but now two pixels down. As you can see, we have an abundance of the line so cut off two pixels on each end. Add a vertical black line of three pixels high in the corner. Merge the layers with the rest of the building. Those innerwalls need some color. Select the Paint Bucket and use the colors of the big outerwalls.
Expanding some more
It still looks a bit boring, so attach some other rectangles on it. You may repeat image 1 to image 4 for this, but use other sizes. Move the rectangles around untill you find a suitable spot for them. Lets make another inlet. we have to make a reversed rectangle. We have to cut out a small rectangle from the big one. Make the line as in image 1 but then small, around fourteen pixels in width. Move it to the corner with around the same height. The transparent red guideline comes in handy for this. Align it with the black top line of the small cube. You now know how high the inlet should be. Merge the line of the inlet with the building, and delete what's in between. Finish it off by adding pixels on the inside. Because there are new corners now, we have to make the lines light grey. After you're done merge the layers with the rest of the building.