The current regulations under the EEC programme states that a minimum 270mm of loft insulation is required.
Some new properties are increasing their level of loft insulation to 300mm. As long as the loft has adequate ventilation, this is fine. You may lose a little storage space but you will save money!
Good quality loft insulation should last decades and will pay for itself with the cash saved from heating bills. Given the relatively low cost and ease of installation of loft insulation, it should be the first place you begin if you want to make your home more thermally efficient. In this respect, it’s easy to install slightly more than recommended to increase the effectiveness of the insulation.