The 51-year-old became a father for the first time five weeks ago after a "fleeting affair" with 32-year-old Tinglan Hong.
According to The Sun, Hong was given the house in Fulham, West London by Grant's cousin Flora Wood, and moved in during her pregnancy in July.
Grant arrived for a 40-minute meeting at the property last night (November 2), where he told reporters that he had flown back from filming in Germany to "protect [his] baby".
However, press reports suggest that the actor was in Germany to meet his new girlfriend Elisa Schmidt, a 21-year-old jazz singer, in Berlin.
The pair have denied rumors of a relationship, though the German media allege that they were spotted "flirting and sharing hot kisses" during a recent night out.