Finishing a basement is usually the cheapest square footage you will ever add to a house — no foundation, no roof, the shell already exists. But New Hampshire basements come with water, cold and code issues that a finished room upstairs never has to deal with, and those are where the budget goes.

The budget bands

For a Monadnock-region basement, installed, permits included:

• Basic finish (one open rec room, LVP floor, drywall, lighting, no bath): roughly $35–$60 per square foot.
• Full finish (a bedroom with egress, a full bath, a wet bar or kitchenette, better finishes): roughly $60–$110 per square foot.
• In-law / rental suite (kitchen, bath, separate entrance, sound isolation): $110–$170+ per square foot.

A typical 700–900 square foot basement therefore runs about $30,000–$55,000 basic, $55,000–$100,000 full. Run your number through the remodel budget breakdown for the line-item split.

Where the money actually goes

• Moisture control first — perimeter drainage or an interior system, a sump with a battery backup, and a dedicated dehumidifier (SaniDry). Skip this and the finish fails. Budget $3,000–$12,000 depending on what we find.
• Egress — any sleeping room below grade needs an egress window and a code-sized well, cut into the foundation. Figure $3,500–$7,000 per opening.
• Ceiling height — you need 7 feet finished under beams and ducts. Low spots may mean a dropped soffit, relocating ductwork, or (rarely) lowering the slab.
• The bathroom — if the drain line is above the slab, a macerating (up-flush) system avoids breaking concrete. Adds $12,000–$25,000 for a full bath.
• Insulation & comfort — rigid foam on the walls, and a plan for heat. A mini-split zone is common.

What surprises people

The finish is the cheap part. Water management, egress, and getting heat and a bathroom down there are 40–60% of the number on a full finish. Get a contractor to look at the foundation, the drainage and the mechanicals before you fall in love with a layout — that walk-through tells you whether this is a $40k project or a $90k one.

The payback

A finished basement recoups a solid share at sale and, unlike an addition, it does not raise your footprint or (usually) your taxes much. If it becomes an in-law suite or a rental, the math gets better again.