Decking material is the biggest cost variablePressure-treated lumber decks cost $4,400–$6,600 in Chicago — durable but require annual sealing or staining and periodic board replacement. Composite decking (Trex, TimberTech) costs $7,699–$19,360 — virtually maintenance-free and holds up well in Chicago's continental climate. Hardwood (ipe, mahogany) costs $9,900–$24,200+ and is exceptionally durable but requires oiling. Match material to your maintenance tolerance.
Deck size and height above gradeDeck cost scales directly with square footage. Ground-level decks (within 30 inches of grade) are simpler and cheaper to build. Elevated decks require deeper footings, more structural framing, and often stairs and railings on multiple sides — adding $2,000–$6,000 compared to a ground-level footprint of the same size. Confirm what height your deck will sit at before comparing quotes.
Footings must reach below frost depth in ChicagoIn Chicago, where harsh winters, hot summers, deck footings must extend below the local frost depth to prevent heaving. Frost depth varies by climate — in colder Chicago markets it can exceed 48 inches. Helical piers or concrete footings that don't reach the correct depth will cause the deck to shift and potentially detach from the house. Ask your contractor what footing depth their quote includes and verify it meets local code.
Permits are required and non-negotiableDecks attached to the house require a building permit in virtually every Chicago municipality — no exceptions. Unpermitted decks are flagged in home inspections, can require removal at sale, and void your homeowner's insurance if someone is injured. Any contractor who suggests skipping the permit should be crossed off your list immediately. Permit fees ($200–$800) should be included in your quote.
Ledger connection to the house is a structural critical pointAn attached deck is connected to your home via a ledger board bolted through the rim joist. Improper ledger connections are responsible for the majority of deck collapses. Verify that your contractor uses through-bolts (not nails or lag screws) and will flash the ledger connection to prevent water infiltration — a leading cause of rim joist rot that undermines the connection over time.
Railings and stairs add meaningful costRailings are required for decks more than 30 inches above grade and cost $30–$100 per linear foot depending on material (wood, aluminum, cable, glass). Stairs add $500–$2,000 depending on run and design. A 400 sq ft elevated deck with perimeter railing and one stair run will have $3,000–$8,000 in railing and stairs alone — make sure these are broken out in your quote so you can compare bids accurately.