Dan Gilbert's Rocket Companies plans to buy the online real estate brokerage firm Redfin, a $1.75 billion deal that would expand Rocket's presence in all things residential real estate. The all-stock deal was announced Monday morning and, pending approval by Redfin shareholders, is expected to close in the second or third quarter.

Redfin shares are surging 70% Monday after mortgage giant Rocket Companies announced it was buying the digital real estate brokerage in an all-stock deal valued at $1.75 billion.

Rocket Cos. said Monday it would pay $1.8 billion in stock for the online real-estate agency Redfin Corp. in an effort to make it easier for customers to buy property using their mobile devices. “We want a customer to be able to check her phone to find out what she can afford,