From 1^st October 2010 the new Part L 2009 Building Regulations come into force. Any building projects initiating after this date are subject to the new regulations. Key features of Part L 2009 are :

• Reduced limiting U values for external walls, floors, roofs and windows. This equates to a 25% decrease in allowable CO₂ emissions compared with Part L 2006.
• Cavity party walls are now treated as a potential heat loss route (U value 0.5 Wm^-2K^-1) unless filled and sealed.
• The requirement for pressure testing for air tightness will roughly double. A pressure test will be mandated on at least 50% or 3 instances of each dwelling type, whichever is the fewer.
• A minimum of 75% of light fittings must now be low energy, All low energy light fittings fitted now count towards achieving the target energy efficiency.
• The default y value for the use of Accredited Construction Details (0.8 Wm^-1K^-1) is no longer permitted. Explicit y values must be calculated by summing each thermal element junction using approved y values for each junction type.