The "How I Met Your Mother" star owns a house in the Hollywood Hills. According to the lawsuit, the prior owner of Radnor's property had a right to extend his property to the neighbor's land, but only if he maintained a retaining wall and walkway.

Fast forward to 2016 ... Radnor, according to the lawsuit, knocked down the retaining wall and built a deck that extended over the neighbor's property. The deck sounds awesome -- seating area and all.

Problem is ... the neighbor thinks the easement went poof when Radnor destroyed the retaining wall.

According to the lawsuit, the neighbor demanded that Radnor tear down the deck ... to no avail.

We reached out to Radnor's publicist, so far no word back.