After building the first part of the house, check it out by going inside. After you have had a look, go back to the Drafting table and add:  Main Hall.

Next, use the Carpenter's Workbench and create:
Main Hall, Door (Requires Iron Fittings, Sawn Log, Lock, 2 Hinge, 2 Nails)

Once you complete the doors, you will build:
Main Hall, Foundation (Requires 4 Sawn Log, 30 Quarried Stone)

Then build:
Main Hall, Floor (Requires 6 Quarried Stone)
Main Hall, Wall Framing (Requires 16 Sawn Log, 20 Nails)
Main Hall, First Level Walls (Requires 4 Sawn Log, 16 Nails, 8 Clay)
Main Hall, Second Floor Supports (Requires 6 Sawn Log, 10 Nails)
Main Hall, Roof Framing (Requires 10 Sawn Logs)
Main Hall, Second Level Walls (Requires 12 Nails, 8 Clay and 4 Sawn Log)
Main Hall, Roof (Requires 25 Nails, 2 Sawn Log)
Cellar (Requires 8 Sawn Log, 50 Quarried Stone)

Congratulations, your house now has a main hall! It should look a lot bigger too, finally looking like a huge manor compared to what you started with.

In the next part of the tutorial, you will be adding wings to your house