Why Expand Upward?
Adding a second story can create substantial new space without using as much yard area as a rear or side addition.
That benefit comes with a more complex relationship to the existing house. The first floor must support the new work, a stair needs to fit into the plan and the roof has to be rebuilt or substantially changed.
Assess the Existing Structure
Structural design and engineering may be required for the actual project. An existing one-story home cannot be assumed to support another level without evaluating the foundation, walls and framing for the proposed loads.
Stairs, Rooms and Upper-Level Circulation
The new stair location affects both floors. It can take space from existing rooms and shape the hallway and bedroom layout above.
Planning should also consider bathrooms, utilities and how the upper level connects to existing systems. Those decisions belong early in the project, before the roofline or exterior finishes are finalized.
Second Story vs Expanding Outward
A rear or side addition may be easier on the existing structure but use more of the lot. Building upward preserves more ground area but changes the whole house vertically.
The right direction depends on the property, existing structure and space program. Home Additions & Major Alterations is the broader page when several expansion options are still being compared.