How do I build a house and children's room?

I purchased a house in Riften and fully upgraded it but I can't make a children's room.

How do I purchase a plot of land in Falkreach? How do i make a crafting bench? And if I do, how do I build a house and a children's room? I have Hearthfire, Dawnguard and Dragonborn.

Solution 1:

There are two kinds of houses in Skyrim - the houses you can buy in the base game, and the houses added in the Hearthfire DLC.

The houses that you can buy that are directly in a city are part of the base game. This includes the house you bought in Riften. The customization options in these houses are limited, basically just adding or removing furniture.

There are three plots for houses that were added in Hearthfire. In order to purchase a plot, you'll need to become a Thane in that region. For Falkreach, this means doing some quests for the Jarl. See the Elder Scrolls Wiki for more details.

You should be able to build a crafting bench for your house once you unlock the plot and build a few rooms.

Solution 2:

You buy the plot of land for Lakeview Manor from the Jarl of Falkreath. You don't need to be Thane (I'm not and I have owned Lakeview for a long time). You need to be level 9+ and have done the Jarl some favours.

If I recall correctly, when purchased, the plot comes with a Drafting Table, a Carpenter's Workbench and a small supply of materials (enough for the small house).

More information: http://elderscrolls.wikia.com/wiki/Lakeview_Manor