You will need: 
  Map Editor - https://www.gta5-mods.com/scripts/map-editor 
  Copy beachmansion.xml to gta5 folder,  then when your are in the game, press F7 and Load beachmansion. 
  or copy to gta5/scripts/autoloadmaps To autoload at start up. 

  HAVE FUN :) 