I want to make a games room underground, between the garage

[andy007]

[color=darkblue][size=18][b]I want to make a games room underground, between the garage and house. The garage flat roof is level with the bottom of the house and about 8 metres away. I was going to fit a door into the garage to lead into the new room and another staircase and door up through the house hallway to get into the house. I will try and keep the grass on top of it so it will not look different from outside. Any idea what it would cost, i can do most stuff myself with a little help from this site.[/b][/size][/color] :?: