Loft Conversion

If you are planning to convert a loft in your house into living space then most probably it will not be subject to the same constraints as a typical house extensions. However, please be aware that certain alterations require planning permission and building regulations approval.
Building Regulations (Parts L, K, B, P)
Like any new building work or significant home alterations, all loft conversions require Building Regulations approval to ensure safety measures are in place. 
Part L
Part L stands for U values. It is important to minimise the heat loss through walls, floors and most importantly, the roof. You can easily achieve this by using any modern insulation, as Celotex for example. Insulation could be added between and beneath the rafters. it could be even added over the rafters, which is know as "warm roof" insulation. This usually involves using 100mm of Celotex (or similar) insulation, followed by a breathable waterproof membrane. When applying insulation make sure no gaps been left between the sheets. Rockwool fibre foam insulation could be used to decrease heat transfer between the loft and the rooms below.

Part K
Part K of the Building Regulations sets standard measurements for living space. This demands a minimum headroom of 2m in all escape routes, including the stairs. This head height can be reduced to 1.8m at the edges of a stairway if allowing for a sloping roof, as long as the centre of the flight is 2m.

Part B covers fire safety. Due to the height of a loft, you can't rely on windows as means of escape. Escaping from upper floors through the stairs should be possible if stairs and doors of all rooms on the way are at least 30-minute fire resistant. 
Part P of the Building Regulations refers to electrical safety. All electrical work will need to be done by a Certified electrician, who would issue an Electrical Safety Certificate at the end of the works.
Planning Permission
Additions of up to 50m³ (40m³ for a terraced home) fall under Permitted Development, but you will need planning permission for any conversion if you live in a listed building or a Conservation Area. You will also need permission if you are altering the roof height or shape, as may be the case if you have to raise it for headroom.
Rooflights and dormers can be installed under Permitted Development, but they must not sit forward of the roof plane on the principal elevation, nor must they be higher than the highest part of the existing roof.