ok here's the SPOILERIFIC info.
Once Hearthfire has been downloaded you can load up your Skyrim character of choice as usual. A courier will approach you after a while with several letters. One is from the Jarl of Falkreath's steward, asking you to come and talk to her about purchasing a plot of land ((for 5000 gold)).
The second letter is from the orphanage in Riften, asking people to come and adopt children, although after a bit of wandering around Windhelm I've found a child looking for adoption by herself and according to the Bethesda forums there are other new children in other cities as well.
The third letter I received was from the Jarl of Windhelm (where Haa-Rei currently resides) telling me that I can add a children's bedroom onto my current home, all holds have this option apparently I only have the Windhelm house though.
Now to the building bit. The first thing you do after buying your plot of land is to go and check it out, it's situated just north of Pinewatch in Falkreath hold, overlooking the lake. It's name, shockingly, is Lakeview manor.
There is a chest and working area already there, the chest contains enough materials to build a small hut. The other materials such as clay, stone and lumber can be mined and purchased in order to expand your small hut into the large estate they you've dreamed of. Oh yes, you also get a lovely Redguard steward from the Jarl of Falkreath as a moving in gift

If you want to leave your small hut how it is then you can, stables, gardens e.t.c are available for crafting straight away.
You also have a sort of pet hawk, it has a nest, containing an egg and some feathers, on the roof of your home.
Stone and Clay quarries are located near your home and can never run out, allowing you to mine forever and ever and ever. In order to get more logs you have to travel to a mill and order some, they get delivered straight to your home. You can just purchase the lumber if you want or you can use the mill to cut it yourself, either way it gets delivered (without postage and packaging

) Your steward can also get materials for you, for a small fee.
Once you have purchased a garden you may plant any plant that you have in your inventory apart from nirnroot, for example I have some blue mountain flowers which I can choose to plant.
FISH HATCHERY: This is unique to the home in Hjaalmarch due to it's close proximity to the ocean, you may build a fish hatchery, any fish or fish egg that you place in here will breed/hatch which gives you a supply of various fish, if you place slaughterfish eggs in here you will get some friendly slaughterfish living and breeding here. It's also a nice addition to the marshland for the aesthetic value
I can now confirm some new foods available for purchase at inns: Argonian Bloodwine, butter, Surilie Brothers wine, sack of flour. (Food items such as soups and stews appear to have been re-textured as well)
New stuff in general stores: Glass, straw, goat horns, childrens clothes, childrens doll, wooden sword (for players and children alike)
**Word to the wise: you're gonna need a heck of a lot of iron ingots (I've used over 200 so far), gold (I've spent around 30k and counting), and corundum ingots (no more than 20 i'd say) for each home to be fully upgraded. You'll also need lots of things like horker tusks, bear pelts e.t.c for mounted animal heads if you want them**
The kitchen extension adds an oven which allows you to do some baking, you can bake: apple dumpling, apple pie, braided bread, bread, chicken dumpling, garlic bread, jazbay crostata, juniper berry crostata, lavender dumpling, potato bread, snowberry crostata and sweet roll.
Mudcrabs now drop mudcrab legs which can be cooked at the conventional cooking pot.
When building extensions you have lots of combos to choose from; you now have the option to add three wings to your home, for each wing there are three additions to choose from, you may only have 1 addition per wing. All rooms have a small crafting bench to add interior upgrades with, you can toggle these benches on or off using the workbench outside.
The west wing can have: bedrooms (although all houses already have bedrooms these ones are nicer), a greenhouse or an enchanters tower.
The north wing can have: storage room, a trophy room or an alchemy lab.
The east wing can have: an armory, a kitchen or a library.
TROPHY ROOM: once you've built a trophy room you can use the crafting table inside to build trophy bases, 3 large and 4 small. The large trophy bases have scale models of: bear, chaurus, cow, deer, dragon skull, draugr, dwarven sphere, falmer, frost troll, frostbite spider, horker, sabre cat, snow bear, troll and wolf trophies to choose from, 1 per base. The small bases have scale models of: draugr, dwarven spider, falmer, goat, hagraven, ice wolf, mudcrab, skeever, skeleton, slaughterfish and spriggan to choose from, again it's 1 per base. The models require items such as timber and straw as well as creature specific items such as dwemer scrap or slaughterfish scales in order to be crafted.
LIBRARY: The library is housed in a decent sized tower, lots of bookshelves are available for crafting, obviously, and you can build a table and chair set on the roof.
ARMORY: This adds lots of display space with 5 mannequins placed on top of 5 small cupboards, 8 weapon racks and 2 weapon plaques, there are also armory stuff such as grindstones available, chests and dressers, and an outside training area with archery targets.
ENCHANTERS TOWER: Like the library it is housed in a tower, the main feature is the arcane enchanter but otherwise there is generic furniture and display cases available as always.
GREENHOUSE: as the name suggests this room is dedicated to growing plants, you can build 3 large planters which can house 4 plants each, then you can build 6 small planters which house 1 plant each. Like the outdoor garden you can grow any plant in here apart from Nirnroot and its crimson cousin.
NOTE: your house also has a cellar which acts as a forge and storage area, there are also barrels of mead which you can use to fill up some bottles if your feeling thirsty

The cellar also has a place where you can build shrines to any or all of the nine divines, each divine's shrine requires different materials to craft.
Your housecarl is pretty important by the way, you can ask them to be your steward and then they can hire a personal carriage and bard, purchase supplies for you to save time and buy things like chickens, cows and horses. They can also provide basic furnishings for your homes for a small fee.
OTHER HOLDS: If Falkreath isn't your style you can purchase a plot of land in Hjaalmarch (Morthal hold) from the Hold's steward. The name of this home is Windstad Manor, it is situated north of Ustengrov, very near to High Gate Ruins right at the edge of the marshes with a view of the sea, and Solitude in the distance. As with the Falkreath home it costs 5000 gold and you receive all the necessary materials to build a basic yet cosy hut.
Still not happy? Then how about a home in The Pale (Dawnstar hold) ((I had to speak to the jarl for purchase options for some reason)). The name of this home is Heljarchen Hall, it is located just west of the giant camp Blizzard Rest, on the border between the snowy mountains of Dawnstar and the flat tundra of Whiterun. Again this home costs 5000 gold and you receive all the necessary materials to build a basic yet cosy hut.
Having three homes gives you the ability to have all the different accessories, for instance one home could have the alchemy and enchanting towers with a library, one home could have a greenhouse, a trophy room and an armory e.t.c
ADOPTION OPTION: Adoption is pretty simple, once you have a child's bedroom in your house (including houses you owned prior to Hearthfire) you can casually wander up to kids and ask them to move in. You can have a maximum of two children.
There are 3 other children added to the game and children at the orphanage in Riften are available for adoption. you can also adopt a child if you kill their parents as the child will be moved to the orphanage.
I have adopted a child from Windhelm named Sofie, she can be found in the gray quarter selling flowers in order to afford food. You simply ask if she wants to be adopted and she happily agrees and runs off to her new home. Once there you can give her gifts such as new clothes, wooden toy swords and dolls which can be bought from any general goods merchant (including your spouse).
You can then play tag or hid&seek with your child, tell them to go play outside or go do their chores and at night time you can tell them to go to bed. It is also possible to give your children weapons such as daggers, to what end I'm not sure.
Your child may also decide to have a pet; foxes, skeevers and mudcrabs have been reported as available pets and your child will just find them nearby and ask to keep them. Dogs in the game, including huskies added by dawnguard, may also become family pets if you wish.