Now lets start with foundations, not every house needs a foundation or would need one to be realistic but if you choose to add foundations here is how i generally like to do them. In medieval times people would build their houses on bases of stone to keep the timber frames from rotting. To start I make a rectangular base an odd number of blocks wide by any number of blocks no more than 3 times the width long. There are exceptions to this, however. If you plan on putting your house near a castle wall or large cliff for example, and you can't accommodate for the 1 block overhang on every side, make the house an even number of blocks wide this way you can make the overhang on one side of the house but not the other. The bases should then be brought 3 blocks high but you can play around with height if you want. These pictures should kind of explain what I'm talking about.

Now, these foundations can be a few different blocks but generally i like to stick to stone brick, cobblestone, or, if you're feeling like it'll work, stone. If you choose cobblestone you should use stone brick as the decor, if you use stone brick, you should to use cobblestone as the decor. this isn't a strict thing i live by but just something i generally follow as it looks best this way. There are a few different styles of decor you can use on the foundations.

Don't worry about windows and doors just yet, we're getting to that. Here are some examples of smaller bases that just elevate the structure slightly

That just about sums it up for foundations, also don't worry about block variation or anything like that yet

There are many many styles you could use for roofing, personally i like to base my roofing style on location and if I'm doing a large village or city i make most of the roofs the same basic style, there are generally 3 basic roof pitches, steep, shallow, and normal but designs can vary. Here are a few designs I use with a house that is 5 blocks wide, however these can also be applied to wider houses

Now obviously these are just skeletons, the color of the timbers as well as the roofing should be dependent on the location (spruce in the taiga, dark oak in darker biomes, etc). With steeper roofing i generally do darker colors and with shallow roofing i do lighter colors, now lets see these roof styles with spruce laid on all of them. Remember, you want a 1 block overhang on the ends as well as a 2 block overhang on the very peak.

now you can see that the roofing that peaks with a full block has a stair as the overhang, this can be faced either up or down, other orientations or blocks that i have tried didn't look as great but be creative with it. Also, you don't need a 2 block overhang if you don't want one, in most cases it adds a bit more depth though.
Now, this is completely optional, but you can then add stairs under those stairs to make it look more "clean" but some would disagree and say it is kind of a thick look, either way is fine in my opinion but it really depends on the style.
Adding detail to roofing is relatively simple and there are many ways to do it. If you are building houses in poorer regions try placing blocks sideways in the middle of the roof to make small holes in the roof. If you want a more natural look try kind of curving the roof in the middle, almost a sagging look that can be applied to both low and high end houses. Its hard to explain this sagging look so just try to see what i did in the pictures. For most roofs especially for poor houses id put sideways stairs on the ends, kind of adds a "frayed" look to it.

You can see in the first image the "frayed" look, in the second image is the sort of curve in the roof, this can be done with any style and you can exaggerate it more or less however you want it.
I often see people build and leave these big plain roofs with nothing on them from end to end. If you feel like your roof is kind of plain add roof dormers. The easiest way to do this is to make a small incline on the side of the house, try not to make it as tall as the original roof but sometimes you may find that you have to, here is an example of a roof dormer on this house.

you can see the same detailing is added to the dormer as the original roof
That is pretty much it for roofing!

Many would argue that walls are the most important part of the house, I don't really think so.. The only thing you really need to do is decide the color you want and the shape of the timber framing, Im not going to cover block variation or details like that in this section, that'll be apart of the detailing section. When deciding your wall color take into consideration the location (once again) and the shade of your roof. If your roof is an oak color you shouldn't make your walls something like sandstone, generally i like to think that colors opposite of each other work well such as a white color with spruce or a brown color with oak. Now for the timber framing what you are going for is a 3 block space between timbers. But if you can go around the whole house with 3 blocks spaces on every wall, don't. Why? well because then it seems too plain, you generally want 3 blocks between timbers but if you do it too much it looks odd so throw in 2 1 block spaces or 1 2 block space here and there. In this build Ill put 2, 1 block spaces and 1 3 block space on one side then 2 3 block spaces on the other side, and 2 2 block spaces on one side and 1 3 block and 1 1 block on the other. I know this seems confusing to read but if you refer to the picture you can see 2 of the sides. The color i went with is a plain stained clay which is kind of a burnt orange. The timbers are also 3 blocks from the base log or 4 blocks from the foundation. This will give us 3 block floors later on. Also i did not include windows as we will cover those next.

There isn't much to my walls really they are very simple, I know a lot of people like doing things like setting the timbers 1 block out from the wall to accommodate more details but personally I think it really cuts down the interior and overall is only a good look from a distance and isn't very functional, I know lots of people like it and I don't mind it either, so do the walls however you feel is best, like i said these are just tips of my own. Here is a picture of a few different block colors i use for my walls.

(some of these are very situational) these blocks are also all the same general color temperature and just a selection from one climate that i have been working on.

Again, many styles of windows you can use on medieval houses, usually i leave windows on houses to one block openings but in many cases 2 blocks can be used for more light and a more open feel. In castles i almost always have at least 2 block by 1 block windows in towers and keeps sometimes even 3 by 1 block windows but in houses i feel like the windows probably weren't very large.

Poorer regions should use something like fence as the actual window to simulate just a simple wood cross without glass. Usually the glass is reserved for higher class areas and when it is used i like to stick to neutral dark colors like brown, gray, or black. Sometimes you can also just leave windows open, this is perfectly ok and makes sense. If you have walls that are 4 by 3 blocks and have no timbers you can also use 2 by 1 windows but generally i just leave them open. A lot of people like to use shutters (trap doors) on the sides of open windows and while i don't mind this look, sometimes it looks odd, so i prefer to have shutters that open upwards. The fence in the windows should generally be a different color than the roof and work well with the wall color, in these pictures the fence is oak but if the roof was oak Id use something like spruce or dark oak again, dependent on the wall color as well.

You can have them above windows with fence in them but usually the glass windows i leave to just glass. For a more natural feel if you have access to world edit //set a block just outside the fence to trapdoor or trapdoor:1 to have a closed shutter. Another thing, if there is space I usually add at most one window to the gables as I have in this house.

Windows can also be added to the foundation, I almost never use shutters if I do choose to do this however usually a darker color fence works nicely.

Decoration is what really brings peoples builds alive, without it most builds just look plain and overall boring. Here I will introduce how I place trees, shrubs, flowers, and other small details.
I usually start with bushes, you can usually get away with using any leaf type however you should not stick to just one type, generally I use something like birch and spruce leaves. start at the base of the house somewhere away from the front door, which you can see Ive added to the side with the roof dormer. Make the base a fair size and try to give it a look as if its crawling up the side of the building. You don't have to do this with all the bushes but one or two makes for a nice look.

From here I like to turn off vine growth and add vines on the leaves and partially on the house

Now add grass, one layer of grass around most of the border of the house looks nice and from there you can add a bit more to the spots that might have been a little more neglected. Taller grass works well against walls and bushes. After the grass is added, throw in a few flowers, I rarely use more than 2 colors, if you use more than 2 colors it starts looking very unnatural. Sometimes it works depending what the colors are but personally I rarely do it. Flowers should be placed near the house but not all against it, sometimes its ok to put 2 next to each other but I wouldn't do it more than once. People in medieval times generally didn't worry about what their houses looked like, especially considering most of their day was spent working (over 12 hours). Btw sorry about the change in textures didn't realize my pack was off until now :P

Now you can add the front door and path, paths are tricky for most people but for me you just need to make a few basic shapes and it will make for a great looking path. I use various blocks such as gravel, cobblestone, coarse dirt, spruce wood (they used to lay wood down on paths because of the mud), and sometimes stone. Then you can throw some grass, maybe a tree in there:

Now you can add things like coal, pumpkins, melons, whatever you think people would have kept outside, I usually have a few blocks of coal and some melons. Hay bales work very nicely as well in rural areas.

Sometimes Ill also add window boxes, these work in some situations especially windows more than 1 block long, I usually fill them with either grass or a bushy looking flower

Other details I like to add are balconies or small overhangs for storage when they seem to fit nicely:

That about sums it up for external decoration.
The texture pack is a custom pack I made that is still a work in progress but will be released soon