Wealthy London ##ers are extending many floors beneath their homes . The aim is to increase their space and provide extra income . Tight space and strict planning laws make expansion upwards or outwards difficult . `` The price per square foot ( of basement extension ) in areas we work in is probably £400 to £500 per square foot ( between $ 660 and $ 830 per sq ft ) . Some have installed swimming pools , gyms and restaurants in their extended basements .
Wealthy London ##ers are extending many floors beneath their homes . The aim is to increase their space and provide extra income . Tight space and strict planning laws make expansion upwards or outwards difficult . Some have installed swimming pools , gyms and restaurants in their extended basements .