Costs for most buildings cover standard building materials, and give you a simple building with limited security. It is good enough for most purposes, but it won't stop a determined burglar and is susceptible to fire. However, you should note that town and city fires are not particularly common, although they can be incredibly devastating. Fortified buildings are more difficult to burgle, and more likely to survive fire and skirmish level violence. Heavily fortified buildings are even more secure and might even survive limited warfare. However, if you think your building will be attacked regularly, or consistently, you should consider adding guards, guard poiints and maybe even a courtyard with a defensive wall - effectively turning it into a small castle.

• Standard - Wood, Lathe & Plaster, Wattle & Daub, Adobe, Sun Dried Bricks. Unless specified otherwise, standard building materials are used for the majority of buildings. They have thatched or shingle roofs, ordinary doors and light shutters at the windows. These building cannot be fortified (Except for Palisades)
• Fortified - Buildings can be fortified by upgrading the walls to field stone, cobbles, kiln fires bricks, or similar materials. Build a Defensive Wall around the perimeter of the building (260gp for forty squares) and add a Fortification Augmentation (300gp). Total of 560gp for an 'average' building. Note this only fortifies the outer shell of the building, if you want individual rooms fortified, you must buy a Fortification Augmentation for each room individually.
• Heavily fortified - a fortified building can be furthered strengthened by adding upgrading the defensive walls to Stone Walls. This costs 260gp per 40 squares perimeter.

Note: If you want a castle, or a really secure home, you will have to add guard posts, a gate house, armoury etc. as well as employ troops to guard the estate for you.