How to plan and cost a loft conversion

Discover the rules, regulations and costs for planning your loft conversion project

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A loft conversion is potentially the most cost effective way of adding an extra room to your home. It is also the most cost effective way of adding value to your property and could be a good investment if you’re going to sell in the future.

One of the most frequently asked questions about loft conversions is whether will it actually add value to your home. There are a lot of factors to consider, including the ceiling value of your street, how difficult it will be for you to get planning permission, and whether your conversion will comply with building regulations.

There is a limit to how much you should spend on your loft conversion. If you plan on eventually selling the house, you will need to consider the ceiling price of your street.

You want the value of your house to increase by at least the cost of your entire loft conversion, but by spending too much, you may over-value your own house, making it difficult to sell for an appropriate profit.

Comparing the quoted cost of your loft conversion, plus the value of your home, with the cost of moving to a larger house in the same area is a worthwhile practice in assessing the benefits of converting the space.

For example:

Your house is valued at £270,000.

The loft conversion costs £50,000, making the projected cost of the home £320,000.

However, the ceiling price of your street is £300,000, making it impossible to recoup the £20,000 excess spent on a loft conversion.

If there is a house in your area with the space you require for less than £320,000, it is worth considering a move, rather than investing in the loft conversion.

exterior of a loft conversion and extension

A dark an cramped loft in an otherwise bright home was converted and extended out to create a wonderful, luxury bathroom

What will a loft conversion cost?

The cost will vary depending on size, but is usually between £30,000 and £50,000. A typical conversion with a rear dormer in a mid-terrace property costs around £35,000. You’ll find small companies will usually charge 10-15 per cent less than large companies.

A Certificate of Lawfulness (£86) or planning fees (£172) are often excluded from the agreed contract with the loft company and are paid directly by the homeowner, so need considering.

Building Control Fees (around £500, plus VAT) are also payable by the homeowner to the local authority or a government-approved, independent inspection company, to check that the work is as contracted and to issue building regulation certificates to prove that it has been carried out in accordance.

Expect to pay around:

  • £1,150-£1,350 per square metre for a basic rooflight conversion
  • £1,250-£1,450 per square metre for a dormer conversion
  • £1,350-£1,550 per square metre for complex options, such

Find a loft conversion specialist

master bedroom an en suite loft convesion with blue feature wall

The loft in this three bedroom house gave the family the potential to create this spacious master bedroom with an en suite

Planning permission for loft conversions

In most cases, converting an existing loft space will not need planning permission, and loft extensions are allowed under permitted development.

Permitted development allows for a roof space increase of 50 cubic meters in detached or semi-detached homes, and 40 cubic metres in a terraced property. Restrictions apply when your home is listed or sits within a conservation area.

A common misconception is that you can build up to the permitted development allowance, then apply for permission for anything over that; but this won’t work. In many areas, the guidance for lofts converted under full planning permission is actually more restrictive than the allowable limit under permitted development, so weigh up what is possible before committing to a design.

Find out whether your project needs planning permission

Building regulations

Always remember that planning permission does not equal building regulations approval – the two have to be cleared separately. Every new conversion, including those done under permitted development, must comply with fire and building regulations. This covers the safety and quality of the building work, including:

  • checking the proposed structure is calculated properly
  • the safety of the stairs
  • insulation levels
  • effective drainage
  • electrical safety

Ask your local authority’s building control department, or a private sector approved inspector, to help early in the planning stages. A common pitfall is the need for a ‘direct means of escape’, so a separated stairway and hallway with fire-rated doors to all rooms, will be required.

master bedroom created out of a loft in a family home

Removing the roof and converting the loft on this 1930s semi detached home created enough space for a master bedroom, en suite and office to the home, adding much needed space for the owner

Designing a loft conversion

Key considerations:

  • Is it the best use of the space?
  • Is there space for furniture and storage?
  • Is the ceiling height acceptable?
  • Could the addition of a dormer create more space?
  • Does the position of the stairs optimise potential space?
  • Could more light be created with roof lights or light tunnels?

When a builder designs your loft, they will generally look for the most cost-effective option that will have least impact upon the rest of the house. However, as long as the construction is structurally sound and the work meets building regulations, there is no harm in exploring alternative placements for stairs and doors.

Usually, the most efficient position will be above the staircase that links the ground floor and first floor, but the best position will depend upon available heights, proposed use, roof shape and the position of the door to the loft room.

When the proposed space is one room, it is often good to locate the door to the space at the foot of the staircase rather than at the top to create an additional sense of space and openness on the new floor. You could also locate the door halfway up a new staircase with a double landing, as you can’t have a door right on a staircase. This gives an open feel at both the top and bottom of the staircase.

It is always worth running your plans and designs past an architect or designer that can visualise them with 3D drawings. The issue with a loft conversion is that floor plans can look large, but don’t take into account the loss of usable space due to sloping ceilings.

Digital models can still be misleading depending on the viewpoint of the particular software. Often a simple paper model will create the best impression of how the space will actually function. Keep in mind factoring the cost of modelling into your design budget to avoid disappointment when the loft is completed.

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