20×18 Deck Cost in Illinois
A 20×18 (360 sqft) deck in Illinois costs $18,000-$28,800 in mid-tier composite with the state's 1.00× labor multiplier (Midwest). An 18×20 deck (360 sqft) is upper-mid — generous for entertaining without fully committing to wraparound complexity. Illinois labor 1.00× national (Midwest baseline). Chicago's permit process is more involved than downstate.
5 finish tiers — 20×18 in Illinois
National $/sqft × Illinois's 1.00× labor multiplier × 360 sqft = total installed cost. Materials + labor + standard railing included.
Excludes permit ($131-$324 typical in Illinois), demolition (if replacing), and exceptional framing for 42″ frost depth. Chicago metro permits $250-400. Frost depth 42″ across most of state.
20×18 framing notes for Illinois
Illinois considerations
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FAQ — 20×18 deck in Illinois
How much does a 20×18 deck cost in Illinois?▾
In Illinois, a 20×18 (360 sqft) deck costs $9,000-$14,400 in pressure-treated, $18,000-$28,800 in mid-range composite (Trex Transcend / Fiberon Sanctuary), and $27,000-$39,600 in luxury composite/PVC (TimberTech AZEK / Trex Signature). Numbers reflect Illinois's 1.00× labor multiplier on 2026-Q1 retail.
What's the cheapest way to build a 20×18 deck in Illinois?▾
Cheapest = ground-level pressure-treated pine with no railings (where exempt under 30″ height) — ~$7,200-$10,800 in Illinois. Add stairs + railings + above-grade framing and the build goes to ~$10,080-$14,400. DIY saves 30-50% on labor but requires permit + inspection coordination yourself.
How long does it take to build a 20×18 deck in Illinois?▾
Permit timeline in Illinois: 2-6 weeks depending on jurisdiction. Construction time for a 20×18: pro crew = 5-8 days for framing + boards + railings + stairs. DIY = 3-5 weekends for experienced DIYer. Allow extra time for permit approval + inspections.
What framing do I need for a 20×18 deck in Illinois?▾
IRC R507-compliant framing for a 20×18 deck typically uses 2×8 joists at 16″ OC for spans up to 12 ft, 2×10 joists for spans 12-16 ft. Illinois's frost depth is 42″ — footings must extend below that per IRC R403.1.4. Sonotube 10-12″ diameter at 42″ depth is typical. Verify with your local building department.
Is composite worth it for a 20×18 deck in Illinois?▾
Over 25 years in Illinois: PT pine = ~$10,800 install + ~$22,500 maintenance (stain every 2-3 yr) = $33,300 total. Mid composite = ~$20,880 install + $0 maintenance = $20,880 total. Composite wins by 59-81% over 25-yr lifecycle PLUS better resale value in Illinois's market.