Average Cost to Build a Deck by Size Deck Size Square Footage National Average Cost Range* 14' x 20' 280 $11,200 � $16,800 12' x 24' 288 $11,520 � $17,280 16' x 20' 320 $12,800 � $19,200 20' x 20' 400 $16,000 � $24,000 6 more rows Cost of Deck Installation: How Much Does It Actually Cost? - Trex Protect trexprotect.com https://trexprotect.com � blog � cost-of-deck-installation-... trexprotect.com https://trexprotect.com � blog � cost-of-deck-installation-... Average Cost to Build a Deck by Size Deck Size Square Footage National Average Cost Range* 14' x 20' 280 $11,200 � $16,800 12' x 24' 288 $11,520 � $17,280 16' x 20' 320 $12,800 � $19,200 20' x 20' 400 $16,000 � $24,000 6 more rows