Use your GDO rights to create a bedroom and an en suite bathroom in the loft

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The loft in a standard house with a pitched roof has sloping sides. So to make a decent living space, you need to replace the slope with a normal-height level ceiling, and that means extending the main wall upwards and getting rid of the slope. In shape terms, it’s like placing a giant ‘Toblerone’ on to that side of the roof. You can do this in the side of the pitched roof facing the back garden. (But read the next box too.)

The new construction mustn't be higher than the highest part of the existing roof.

You can only take advantage of the 40/50 extra permitted metres of loft space, if the GDO rights have not already been eaten up by earlier extensions anywhere in the house.

You have to start with the cubic content of the original building. If it’s an old building, it may be hard to work out what the original shape was, but the rules say you can ignore anything done before 1948. so ‘original’ for an old house means how it was in 1948. Taking your new proposed works and any previous works (after 1948) into account, the overall ‘lifetime’ limit on extensions is as follows:

• The resulting house cannot be more than 10% (for a terraced house) or 15% (for detached or semi-detached houses) larger than the original house in cubic contents (unless the next point applies)
• You can always built up to 50 extra cubic metres (for a terraced house) or 70 cubic metres (for detached or semi-detached houses) even if that is more than 10% (or 15%).
• However, never mind what the 10/15% or 50/70 cubic metres rules say, you cannot have more than a cumulative 115 cubic metres of extra space.

Here is an example. You have a Victorian terraced house which was 450 cubic metres (including the inside of the pitched roofs) when it was built. A rear extension was constructed in the 1930s which increased the size to 550 cubic metres. So the original house is 550 cubic metres. You are allowed to extend the houses by 55 cubic metres (10%). It is better than the minimum 50 cubic metres because the 10% allows you to make it 55 cubic metres. But your extension must still keep to 40 cubic metres, because that is the rule for each individual roof extension.

Let us imagine, however, that there had already been an extension of 20 cubic metres in the 1970s. That would reduce the ability to have any further extensions under GTO rules to 35 cubic metres. You can still do a loft conversion, but now you must limit the size to 35 cubic metres.

You aren’t allowed to build out through the slope on a side facing a highway, so you can’t do it on the front/street side. (You can still put in a Velux window because it doesn't stick out of the line of the slope, which is what the planning rules are trying to avoid.) ‘Highway’ can mean pathway, so you may have a problem if there is an alley running at the back of a terrace of houses – ask the Council.

You should also note some building regulations which can have an impact on what you plan to do. Building regulations govern how you do building works, as opposed to planning rules which relate to what building works you can do. Under the building regulations, you cannot use a loft conversion as living space unless there is at least 2.3 metres room height in the centre of the room, and 2 metres over the stairs. Remember that under GDO rules (see above) the roof extension is not allowed to exceed the height of the highest part of the existing roof. So if the existing roof is very shallow - as it often is in Victorian houses - then these building regulations would prevent you from doing the loft conversion, because you wouldn’t be able to provide a permissible room height.