The current regulations under the EEC programme states that a minimum 270mm of loft insulation is required.
The recommended depth for loft insulation is 270 millimetres for glass wool, 250 millimetres for rock wool or 220 millimetres for cellulose.
Any existing insulation can be left in the loft and an additional layer added to it to bring it up to the required minimum thickness. It does not matter how long the existing insulation has been in the loft it still retains its insulant value.
If you are doing a re roof or working on over 25% of a roof and building regulations are required, insulation will be the first thing they look at and they will fail the roof should it not be as per regulation.
Please inform your customers of any potential changes or upgrades.