How thick should loft insulation be?

Loft insulation minimum thickness

The recommended minimum depth of insulation for lofts is 270mm. If you are applying new insulation over old you will have to measure the depth of the old to determine how much is needed to bring it to that amount. Example: If the insulation installed measures 100mm, the insulation applied over it will at least have to measure 170mm.

Loft insulation regulations 2021

The building regulations are aimed at the renovation or replacement of thermal elements, such as walls, floors, or roofs, which separate heated or cooled areas from the outside, or an unheated part of the same building, or a structure exempt from the building regulations, such as a porch or conservatory.

These apply where there is replacement or increase to the thickness of insulation to over 50% of the roof.

In roof spaces without insulation, you will need to provide thermal insulation to achieve a U-value*  of at least 0.16W/m2K. For example, this would be 250mm of mineral wool loft insulation.

If adding insulation to pre existing insulation the thickness will have to be measured to calculate what is needed to reach the right u value.

No matter if the local authority is notified or not, the authority retains the power to compel the builder or owner of the building to comply with the building regulations. During a building survey, the level of thermal insulation performance can also be questioned, which can also disrupt the sale of the property.

https://www.labc.co.uk/news/what-are-building-regulation-requirements-installing-loft-insulation

Can you have too much loft insulation

The only way you can have too much insulation is if you do not have the adequate ventilation necessary to prevent damp occurring. 

The amount of insulation you use will have to have enough space that it can be installed correctly but also have enough air ventilation to stop the build up of moist air.

Click on the link to read about Boarding a loft over insulation