Using the franchise tag on Richardson would cost the team around $14 million, per Marc Sessler of NFL.com.
The Seahawks acquired Richardson in a trade with the New York Jets just before the start of the regular season, sending wideout Jermaine Kearse and a 2018 second-round pick the other way. Richardson, 27, responded with a solid season, registering 44 tackles, two fumble recoveries, a sack, a forced fumble and an interception.
Pro Football Focus gave him a grade of 83.8 on the season, the 27th-best mark amongst interior defensive linemen.
After the season, Richardson said he wanted to return to Seattle.
"I definitely expect to be back here," he said in January, per Curtis Crabtree of Pro Football Talk, adding: "They haven't threw out a number or nothing but they said they wanted me back so that's a plus in my book."
Head coach Pete Carroll also expressed an interest in retaining the player.
"If we could be fortunate enough to get Sheldon back, that would be huge," he said, per Crabtree.
The Seahawks will have Jarran Reed, Nazair Jones and Malik McDowell available at the position, however, leaving them with some flexibility and leverage in any contract talks. On the other hand, Richardson is an experienced and proven player on a defense that could be facing the possibility of missing veterans like Cliff Avril and Kam Chancellor because of neck injuries.
A reunion makes sense for both sides, but with the Seahawks projected to have around $14 million in cap space, it's certainly feasible the Seahawks could lose out to higher bidders in free agency if they indeed eschew using a franchise or transition tag.