"Big Construction" (大工事, Dai kouji) is an extra chapter included with Volume 7.
Prior to the chapter itself, 2 Omakes are shown.
After killing the aliens, Sweet Mask prepares to go back to his TV shoot, but is stopped by Metal Bat who prepares to fight him. Both their phones then ring, and while Sweet Mask ignores his, Metal Bat answers his. On the other side of the phone is Metal Bat's sister, who placed third in a piano concert, which Metal Bat was supposed to go to. Sweet Mask then prepares to leave once more, but is asked by Metal Bat to give him an autograph for his sister.
Saitama is bothered by a rock in his boot, one that he got from the moon. He then asks Genos how much a moon rock is worth, who tells him one gram is worth 600,000¥. Saitama then hurries to go sell it, but accidentally drops it in a nearby parking lot. Saitama and Genos spend all night, unsuccessfully looking for the rock.
Two days after the destruction of A-City in the Hero Association Headquarters, a staff member talks about how they are planning to expand the building and create highways to allow fast travel, he also claims that it would take 10 years, and that since it would be impossible with normal means, the entire fund has been entrusted to one man. An earthquake is felt, and a man enters the room warning everyone of impending danger. A video feed shows a giant machine, as tall as the Headquarter itself, walking on four legs. The association staff decides to call the heroes for help, and the nearest ones are the Tank Topper Army who are training in the association gym. Before they are alerted, Tanktop Master tells his subordinates, that after losing to Tatsumaki in terms of power, he has decided to keep training, to bring out more "Tanktop Power".
The Tank Topper Army is alerted and left to deal with the threat outside, only to be dumbfounded by its size. Tanktop Master then rushes in and tries his best to stop it. Meanwhile, the machine is extracting small orb-like choppers, which most Tank Toppers are unable to defeat, except for Tanktop Master. The choppers continue to ignore the Tank Toppers and begin to eat the rubble left behind after the destruction of A-City. A voice is then heard from the giant machine, which Tanktop Master recognizes as Metal Knight. Metal Knight tells Tanktop Master that he was requested to rebuild the Hero Association Headquarters and that he shall finish in 10 days. Tanktop Master realizes the amazing feat Metal Knight is about to accomplish and considers it another defeat.
Later on, Tanktop Master is seen continuing training even harder, as he was defeated twice, by both machines and psychic power. Meanwhile, the Hero Association staff members are bothered by Metal Knights methods and are considering him a threat.
Lastly, it is shown that the work that was thought to take 10 years, Metal Knight finished in 7 days.