how big can my loft conversion be
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How big can my loft conversion be?

Loft conversions can be as big as your current roof or bigger, if you decide to modify the structure of the roof or build an extension with another roof on it.

Having a roof with a large area is a good start, but unless you change the structure of the roof, you will always have a relatively small usable are because of the sloping sides.

Building a loft extension with dormers or even a mansard will greatly increase the usable space and give you the biggest loft possible.

The size of a loft conversion will usually limited by the following things:

  • Budget
  • Size of existing property
  • Total Area of land around the property
  • Planning Constraints


The cost of a basic roof light conversion is likely to be around £25k for most houses, so if you want to go really big, the sky will need to be the limit for your budget too.

The best value loft conversion in terms of usable space /£ is a dormer conversion, but if you want maximum space then it will be a mansard conversion, which is essentially a whole new roof and likely to be in excess of £40k.

Those figures are for a typical house; if you have a very large house, that figure could increase by a lot.

Size of Existing Property & Land Around It

The maximum size of a loft conversion is ultimately going to be limited by the size of the building under it.

If you are planning to build an extension to your property at the same time as having a loft conversion, you could have the roof of the extension built as a habitable space and extend the size of your existing roof too, as long as your property is the right size and shape to do it.

As a benchmark, local planning authorities will limit the size of any new extensions to only take up 50% of the land around your original house, so the theoretical maximum size of your loft conversion is the area of your current roof plus the area of 50% of the land around your house, assuming you can afford a double height extension of that size too.

In order to get the maximum usable space inside your loft conversion you will want at least dormers in every sloping portion of roof, if not a full mansard conversion. This is likely to be an expensive option, but that may not be only issue, because you will also need to get it passed by your local planning authority.

Planning Constraints

It is possible to build a loft conversion without getting planning permission under what is know as ‘Permitted Development’ which you can read about here.

There are several criteria, but the general rule is that if you build a loft conversion with a dormer on the rear side of your property then you don’t need planning permission.

If you want to add dormers or a mansard at the front, you will need planning permission. You will also need planning permission if you decide to go for a double height extension, assuming that your house has 2 floors.

Getting planning permission for dormers at the front of your house and large double height extensions is possible, but it will depend a lot on where your house is and how close you are to your neighbours.

Councils and planning authorities tend to take a dim view of extensions or loft conversion which:

  • Block out a lot of light for a neighbour
  • Give the property owner a better view into a neighbour’s property

If you would like advice on what will an won’t be acceptable to your local planning authority, please give us a call.

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