Additions are envelope-heavy — that's why $/sqft is higher than remodels
A room addition builds new foundation, new framing, new roof, new siding, new windows, and new mechanical systems before any interior finish goes in. Roughly 40–55% of the per-sqft budget on an addition is envelope and structure — line items that don't exist on an interior remodel of the same square footage. That's why a $50,000 standard kitchen remodel ($250/sqft × 200 sqft) and a 200 sqft addition at $44,000 ($220/sqft) include very different work for similar money.
Per-sqft cost actually drops as addition size grows. The fixed costs of design, engineering, foundation crew mobilization, and permit hassle get amortized across more square footage. A 400 sqft addition typically costs less per sqft than a 150 sqft "bump-out" that pays the same fixed overhead.
See also: Attic Conversion Cost — $50–$225 per sqft. Cheaper than a full addition when the attic structure can carry living-space load.
Foundation type drives 12–18% of total — and depends on your soil
Slab-on-grade is the cheapest foundation when soil and frost depth allow. Crawlspace foundations cost more but make plumbing and HVAC service vastly easier. Full basement foundations on additions are rare except where the existing house has a full basement and matching the foundation depth is structurally required. On sloped lots or expansive clay soils, pier-and-beam or helical-pier foundations are sometimes the only feasible option and can add $8,000–$25,000.
A geotechnical evaluation runs $1,500–$4,000 and is worth doing on any lot where you don't already have known soil-bearing data from the existing house. Skipping it is how you find out the addition needs deeper footings on day three of excavation.
Roof tie-in is where leaks come from on the cheap additions
Where the new roof meets the existing roof is the single highest-risk water-intrusion point on an addition. Done right, it involves removing existing shingles back to bare deck on the affected slope, weaving in step flashing where the new wall meets the old roof plane, valley flashing where new and old planes intersect, and self-adhering ice & water shield at every transition before any new shingles go on.
A "saddle" tie-in (where the new roof simply butts up to the existing) is shortcut work that fails within 5 years. Specify in the contract that all shingles within 4 feet of the tie-in get replaced and that step flashing and valley metal are new, not reused. The detail adds $400–$1,500 and saves you a future ceiling repair.
Compare: Garage Conversion Cost — $60–$200 per sqft. The other "use existing footprint as new living space" option.
HVAC capacity is the easiest thing to under-budget
Most existing residential HVAC systems are sized for the existing house, not the existing house plus 400 sqft of new conditioned space. Adding 400 sqft typically requires either a system upsize (replacing the air handler or condenser, $6,000–$12,000) or adding a dedicated mini-split for the addition ($3,500–$7,500 installed for a single-zone unit).
Mini-splits are usually the right call. They give the addition independent thermostat control, avoid the cost of redoing duct sizing for the whole house, and don't leave you with one room that's always 4 degrees warmer than the rest of the house. Get the heat-load calc (Manual J) before specifying — eyeballing capacity is how you end up with an undersized system in year 2.
Adding a bath or kitchen pushes the addition into the upscale tier
A 400 sqft addition at standard finish without plumbing is roughly $88,000 base. Adding a full primary bathroom inside that 400 sqft adds $20,000–$45,000 depending on finish — pushing the same footprint into the $108,000–$133,000 base range, $124,000–$153,000 with contingency. A wet bar or kitchenette is similar order of magnitude on the high end.
These wet additions are also where regional multipliers stack hardest. The same primary-suite addition on the Pacific (1.09×) versus East South Central (0.93×) is a $20,000+ swing on the same plans.