Converting the loft of a house is considered to be permitted development (not requiring planning permission) subject to the following limits and conditions:
1. The total area of all floors in the building, including any new story created within the original roof space or by conversion steps/ladders, must not exceed 30 square meters [3 x 10 sq ft] measured from any floor level. This applies regardless of whether this amount exceeds the general permissible developments limit for dwellings of 40sqm.
2. No more than half this total can take place in lofts that were used as bedrooms before 1 July 1997 [AFAIK no such cases; looks like an attempt to limit loft extensions into attics].
3. The entire floor area within the existing roof space can be converted only if it is occupied entirely by plant machinery or fixed storage and equipment.
4. No more than half of any floor in the building can be created at a level more than 4m above that of an adjoining ground floor, nor more than three floors above that level.
5. For buildings within 3km of a designated Ancient Monument no part of the proposed new construction can exceed 6m in height measured from ground level.
6. The maximum eaves and ridge height is 2m higher than that on any other story which does not comprise a terrace or balcony [the latter are allowed to project 1m over the general rule in dwellings].
7. No part of the proposed new work can extend beyond a wall that is not permitted development [see below for interpretation].
8. The conversion must not result in any first-floor window being less than 2m from a boundary or more than 3m from a neighboring property [1m and 2.5m AFAIK]. Also, windows within 2m of the ridge must be wider than 1sq m and those within 5m must be either double-glazed or larger than 1 sq m [AFAIK both sides are covered by general dwelling rules allowing “up to” 80% of floor area as windows plus doors].
9. Any new loft space should comply with building control rules and relevant fire safety regulations.
10. A balcony can be created if the proposed development does not result in a loss of any existing designated parking space [AFAIK no such cases].
11. If you live within 1km of the boundary of an Area of Outstanding Natural Beauty or National Park, there are additional limitations on loft conversions involving dormers that protrude from a roof plane [see below for interpretation]. If in doubt consult your local planning authority before applying to them for consent.
The following table summarises permitted developments by area limits and intervention. (For many developments, the maximum is increased when they take place in certain conservation areas.)