Lagonisi is the southern stretch of the Riviera, running into Saronida. The reason to come this far is space. The gardens are larger, the pools are often the point of the house, and the evenings are quieter than anything in Glyfada. You trade the ability to walk to a shopping street. Most guests who choose it have already decided they will eat at the house more than they eat out.
It is the first place we look for a large group and for a celebration that needs a terrace rather than a ballroom. Not every house allows either. The proposal says so before money changes hands. A private chef is the usual way to feed twenty people here. The kitchen has to be real.
The beach is part of the same southern coast described in the beaches guide, together with Varkiza. Varkiza is the closer sandy option if the house itself is set back. The lake and Astir are a drive north, via Vouliagmeni, reasonable for a day and a poor plan for every morning.
Driving times belong in the decision. Athens International Airport is typically forty minutes or more, and the coast road on a Sunday in August can double the last part. Book the transfer. The Athens Riviera guide is the map of what you are giving up, and what you get, by coming this far south.