Treat design as an early, high-leverage affordability decision. Size, plan efficiency, and complexity multiply costs across structure, envelope, MEP, and finishes.
Builds on: A1 (cost elements and baseline) · Leads to: A6 (materials/labor), A7 (alternative construction) · Cross-series: P5 The House as a System
P/A bridge: Compact form reduces envelope area, transition count, thermal bridges, and labor risk at the same time.
By the numbers
Interior finishes are the #1 construction cost category at 24.1% ($103,391) - larger than framing, larger than MEP. Cabinets alone are 4.5%. [NAHB 2024]
At $162/sf, each 100 sf reduction saves ~$16,200. A 1,600 sf home costs ~$259,200 to build vs. ~$429,000 for 2,647 sf. [NAHB 2024]
Entry-level home production (<1,400 sf) has collapsed 84% since the late 1970s - from 418,000/year down to 65,000. Their share of new construction fell from 40% to 7%. [Freddie Mac 2021]
Core Concepts
Size multiplies cost across most systems. Can quantify which systems scale with size for the baseline project.
Livability often comes from layout efficiency, not more area. Can compare plans by usable function, not just square footage.
Complexity creates cost via labor hours, errors, and rework. Can identify complexity drivers and remove them intentionally.
Cost Elements
Building construction - size (B07-BuildCost) - Size drives structure, envelope, MEP sizing, and finishes. Every additional square foot cascades into every subsystem.
Risk: "Small" decisions of a few hundred extra sq ft cascade into every trade.
AF: B07 - CRO-SIZE, CRO-SIMPLIFY
High-Leverage Plan Moves
Stack plumbing vertically - Align kitchen/bath wet walls across floors. Reduces pipe runs, fittings, and labor hours significantly.
Simplify the roofline - Fewer valleys, hips, and dormers = fewer transitions, less flashing, faster framing, fewer leak opportunities.
Compact the envelope - Reduce exterior wall area per sq ft of floor. A simple rectangle encloses more space with less wall than an articulated plan.
Barriers & Levers
CRO-SIZE
MIN_UNIT_SIZE - Minimum dwelling unit size prohibits smaller, lower-cost homes. City council, planning
PARKING_INDUCED_SIZE - Parking requirements indirectly enforce larger units via garages. Municipality