How much does a mansard loft conversion cost?

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The average cost of a mansard loft conversion ranges between £50,000 and £75,000. Our handy guide covers the different costs to consider. All prices and estimates in this guide are accurate as of 2024.

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Typically built to the rear of a property, a mansard conversion has a horizontal roof with a rear wall that is slightly off the vertical (at least 72 degrees). Mansard conversions can create a great deal of additional loft space, essentially a whole extra floor, but are usually more expensive than other types of loft conversion. In this guide, we will help you plan and budget effectively for your mansard loft conversion, exploring the different factors that will impact the final cost and how best to prepare.

We'll cover the following topics in this pricing guide:

  1. Average mansard loft conversion costs
  2. How long does a mansard loft conversion take?
  3. How to reduce mansard loft conversion costs
  4. FAQ
  5. Your mansard loft conversion checklist

Average mansard loft conversion costs

There are many different stages and tradespeople involved in a mansard loft conversion, from erecting scaffolding through to painting and decorating the interior.

Our cost calculator provides some average costs for the various elements involved:

Mansard loft conversion jobAverage cost
Mansard loft conversion on a small, terraced house£50,000 to £55,000
Mansard loft conversion on a medium-sized semi-detached house£55,000 to £65,000
Mansard loft conversion on a large, detached property£60,000 to £75,000
Scaffolding£4,000 to £5,000
Insulation£800 to £1,200
Windows£700 to £1,200
Flooring£750 to £1,500
Bathroom suite£4,500 to £6,500
Staircase£2,000 to £3,000

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How much does mansard loft conversion labour cost?

A mansard loft conversion is a substantial job, and labour is usually the biggest single cost involved. You’ll need specialists in various areas, including scaffolders, builders, plumbers and electricians. 

We’ve compiled a handy table to help you calculate the labour costs for a mansard loft conversion. 

Builder£25 to £35 per hour or £160 to £300 per day
Plumber£35 to £45 per hour
Electrician£55 to £100 per hour
Plasterer£30 to £55 per hour
Decorator£25 to £40 per hour

Note that labour is usually more expensive in London and the South East.

Cost of mansard loft conversion plans

You will need to use an architect, an architectural technician or a surveyor to draw up plans for your mansard loft conversion.

Typically costing in the region of £1,000 plus VAT, these plans should take all of the structural alterations needed into account.

Cost of planning permission

Unlike most loft conversions, which are typically covered by permitted development rules, mansard loft conversions usually require planning permission because of the extent of the work involved.

You can apply to the planning department of your local authority for permission before you begin work.

If any objections are raised, you may need to make changes to your plans and resubmit them. Applying for planning permission currently costs just over £200 in England and Wales and £300 in Scotland.

Cost of a party wall agreement

If you’re planning a mansard loft conversion and live in a terraced or semi-detached house, you will need to use a specialist surveyor to draw up a party wall agreement. Party wall surveyors usually charge between £100 and £400 per hour. If your neighbours want to use a different surveyor, you will have to pay their costs too.

On average, it costs around £1,500 to draw up a party wall agreement.

Cost of scaffolding

Erecting scaffolding is a crucial first step in a mansard loft conversion. Scaffolding usually costs around £750 to £900 per week, or around £4,000 to £5,000 for a standard six-week period. Your builder will usually include the cost of this in their quote for the whole project.

Cost of loft insulation

Properly insulating your new loft space can help you to reduce the cost of your heating bills. For standard blanket loft insulation, you can expect to pay around £20 per m², while sheet or foam board insulation is more efficient but more expensive at around £30 per m².

Overall, the average cost to insulate a mansard loft typically sits between £750 and £1,000 including labour.

Cost of windows

Windows on a mansard loft conversion usually sit within a box-shaped dormer that projects slightly from the new roof structure. As the windows are positioned vertically, it helps to ensure the loft conversion looks in keeping with the original building. You can expect to pay between £700 and £1200 on windows for your mansard loft conversion.

Cost of a staircase

You’ll need to fit a staircase in order to gain access to your new space. For a standard staircase, you can expect to pay between £1,800 and £2,500, plus the cost of labour. Custom-built options, alternating treads and spiral staircases can cost anywhere up to £10,000.

Cost of a new bathroom

If your mansard loft conversion includes a bathroom as well as a bedroom, you can expect to pay between £4,500 and £6,500 for fitting it out with a shower, toilet and sink.

How long does a mansard loft conversion take?

If everything runs according to plan, a mansard loft conversion should take approximately eight to ten weeks.

Here are the timeframes for some of the common jobs involved:

Erecting scaffoldingOne day
Modifying the roofOne to two weeks
Fitting windowsOne day
Installing a staircaseTwo to three days
ElectricsTwo to three days
Fitting flooringOne day
Fitting a bathroomFour to five days
DecoratingThree to four days

How to reduce mansard loft conversion costs

Keen to keep a lid on the cost of your mansard loft conversion? Hiring a builder who’s registered with the Competent Tradesperson scheme means you won’t need to apply for Building Regulations approval. Here are some other ways to save money:

  • Use second-hand materials: Consider using second-hand materials like reclaimed floorboards or upcycled furniture. Note that any reclaimed materials should still comply with safety standards.
  • Do it yourself: Once the structural work on your mansard loft conversion is complete, you can save money by taking on some of the decorating work yourself, like wallpapering, painting, and installing flooring.
  • Shop around: We recommend getting several quotes for every aspect of the project for comparison so that you can choose the best one to fit your budget.


What permission do I need for a mansard loft conversion?

Most mansard loft conversions require planning permission, so you will need to apply to your local authority for this.  

If you live in a conservation area or your building is listed, you may struggle to obtain permission unless there are similar loft conversions nearby.

If you live in a terraced or semi-detached house and share a wall with a neighbour, you will also need a party wall agreement before carrying out a mansard loft conversion.

Will a mansard loft conversion add value to my home?

As it adds substantially more space to your home, you can expect a mansard loft conversion to increase the value of your home. However, the housing market can be unpredictable, and you should prioritise the quality of the work and obtaining all necessary consent to have the best possible chance of adding significant value when it comes to reselling your home.

What time of year is best for a mansard loft conversion?

Spring or summer are the best times of year for any loft conversion. The days are longer, and the weather tends to be drier, meaning work can go ahead uninterrupted. There is typically more demand for builders at this time of year, however, so we recommend booking in the work as far in advance as possible.

Your mansard loft conversion checklist

  • Do your research: Look carefully into the different types of loft conversion and the costs involved in each. Shop around and get a few quotes for comparison.
  • Factor in extra costs: Make sure your overall budget includes hidden costs like equipment hire, inspections and planning applications. 
  • Complete the necessary paperwork: Make sure you have obtained any necessary planning permission, that the work complies with Building Regulations, and that you have a party wall agreement in place if necessary. Your loft conversion specialist can advise you on exactly what paperwork is necessary.
  • Find a loft conversion specialist you trust: Hiring a reliable and experienced loft conversion specialist can help ensure your mansard loft conversion runs smoothly and stays within budget. MyBuilder carries out rigorous checks to ensure all applicants to our network comply with our hiring requirements. When you post your job on MyBuilder, you can be confident in finding tradespeople you can trust.

Share your mansard loft conversion job on MyBuilder and we’ll share it with reliable and recommended loft conversion specialists in your area. For peace of mind, you can check out feedback from other homeowners and look at accreditations and work histories before deciding who to contact for a quote.