The average mansard loft
conversion costs around £55,000
Pricing guides

How much does a mansard loft conversion cost?

The mansard loft conversion costs in this article are correct as of 2022

Thanks to the dramatic impact they make to your home, mansard loft conversion prices are typically higher than other loft conversion costs, dictated by two main elements:

  • The size of the property - is is a small terraced house, or a large detached property? 
  • The quality of the finish, particuarly any extras, such as if the conversion includes adding a bathroom.

Overall, we estimate mansard roof extension costs will end up between £45,000 and £76,000, with prices usually higher in London and the south east. 

We've broken down three possible scenarios based on size and finish:

  Two-bed terrace Three-bed semi Four-bed detached
Basic £45,000 £55,000 £70,000
Bathroom £2,000 £4,000 £6,000
Total £47,000 £59,000 £76,000


Before you go ahead with such a major project, it’s important that you get a good idea of how much a mansard loft conversion costs. We spoke to some of the expert tradespeople at MyBuilder to find out all the things that can help you budget.

Mansard loft conversion calculator

As above, there are two main factors to calculate in a mansard roof extension:

  • Size of the conversion
  • The finish of the conversion and any extras

Because a mansard loft conversion involves changing the entire sloping face of the roof to a near-vertical angle (at least 72 degrees), it means structural change to the entire roof, so the bigger your home, the greater the cost.

With the finish, if you choose to have a bathroom installed for example, you have to pay for the cost of the suite and other elements in the finish, which can add several thousands of pounds to the overall cost of the project.

Other mansard loft conversion costs can be broken out from this basic calculation.

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Stages of a mansard loft conversion

There are a number of stages involved in completing a mansard loft conversion - you can expect to encounter all of the following if you take on the project, as well as their associated costs.


Mansard loft conversion plans cost

Plans for your mansard loft conversion will need to be drawn up either by an architect, or by an architectural technician or surveyor.

These plans should take all of the structural alterations needed into account, and can cost in the region of £1,000 plus VAT.


Planning permission costs 

Because of the extent of work carried out in constructing a mansard loft conversion, planning permission is usually required, even though this is not the case for a majority of loft conversions, which are covered by permitted development rules.

You will need to contact the planning department of your local authority to apply for permission to make the changes, and have them approved before you can begin work.

You may need to make changes to your plans if any objections are raised and resubmit your plans. Applying for planning permission currently costs just over £200 in England.


Building regulation costs 

Regardless of your planning status, your mansard loft conversion will have to be inspected by the building regulations department of your local authority to ensure it is being constructed in accordance with the rules, making sure it complies with things like fire safety, access, floor strength and so on.

Submitting the plans for inspection and having on-site inspections can cost between around £400 and £800.

You may also need a Party Wall Agreement if your work will impact any adjoining properties, for example if you live in a terraced house or a semi-detached home. This requires your neighbours to agree to the work taking place. You are liable for costs such as their surveyor and any damage to their home which may occur as part of the works - it can cost several thousand pounds depending on how complex the agreement is.

Another potential cost, though not always needed, is a bat survey if you believe your loft may have bats - this can cost up to £400.

MyBuilder Top Tip

Mansard roof conversions are most typically found in terraced houses, particularly in London, where space is at a premium - however, they can added to most houses, including detached and semi-detached houses.

Given the greater space they provide when compared to more common dormer loft conversions, it is worth considering the option if your budget can stretch to it.


Mansard loft conversion building costs

Once your plans have been finalised and approved, initial work on your mansard loft conversion will be carried out externally, with the use of scaffolding and in some cases a roof cover to allow work to continue effectively even with bad weather. 

The roof will be removed, the side walls will be raised, and the new roof will be constructed at a new angle, often with dormer-style windows installed.

At some point, access will be created into the loft from the interior and the floor will be strengthened. Interior walls will be then be fitted along with insulation, and features such as the staircase will be installed, followed by windows.

Electrics and plumbing will go in, before it is plastered and carpentry such as skirting boards are fitted. Electrical sockets and elements like the WC and sink, if needed, are then added, before the final conversion is ready for final inspection, and decoration can then take place.

This building process is where the bulk of the material and labour costs are spent.


Other mansard loft conversion costs

The standard of fixtures and fittings in the loft will make a big difference to the cost of the project - for example, if you are simply planning to use the space as a large bedroom, it will be cheaper than if you are putting a bathroom in the space, where you will need to purchase the suite, which can cost several thousand pounds.

Moving the property’s water tanks if needed, and altering the rest of the home’s plumbing system, will also add to the overall price.

A slate roof will usually cost more to alter than one with concrete roof tiles.

Another factor is whether the roof was built with a frame - often found in pre-1960s homes - or with wooden trusses, which are easier to construct but can take less weight. Replacing or reinforcing the trusses can take more work and thus increase the costs.

How long does a mansard loft conversion take?

For a mansard loft conversion, around six to eight weeks is a typical timeframe for the the actual building work. However, the planning stage can also take a considerable amount of time, especially if planning permission is required.

You will usually be able to stay in your home, without too much disruption, for the duration of the work.

How to keep your mansard loft conversion costs down

The best piece of advice for keeping costs down is hiring experienced and reliable tradespeople. At MyBuilder, we typically advocate hiring particular specialists for each role, however, with mansard loft conversions, there are a number of ways of hiring the necessary tradespeople.

You could hire an architect to plan and let them oversee the entire project, hiring a team of builders and other necessary trades, or hire a specialist loft conversion firm who will have a team with all the needed skills.

You can also project manage the work yourself, hiring different tradespeople for different elements of the build, but it will require a lot of attention to ensure each stage runs to schedule and does not cause delays.

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