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The cost of a mansard roof loft conversion

The cost of a mansard loft conversion will depend ultimately on the size of the property and the detail of the exact works so how you choose to fit out the new space but from scratch, (so not adding a mansard to an existing loft conversion), the cost is on average between £45,000 and £55,000. The bulk of the mansard loft conversion costs are actually spent internally so moving and re-routing water, plumbing and electricity and fitting out the conversion particularly if you are adding a bathroom or kitchen.

Last Updated: October 2022

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If you are considering a loft conversion or extending the loft conversion you already have then a mansard loft conversion might be one of the options on your list.

A loft conversion can completely transform your home, adding valuable extra accommodation without the need, cost and upheaval of building an extension.  The possibilities with a loft conversion are truly endless creating a large open plan area for a studio or bedroom and en-suite, or more than one room for useful extras like a home office and another bathroom.  A mansard loft extension is usually more expensive than a hip-to-gable conversion or a dormer because of the work involved but they do create a vast amount more usable space and stylistically can really make a design statement because of the scope and possibilities they offer.


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What is a mansard roof loft conversion?

The mansard loft conversion is named after the 17th-century French architect, Francois Mansard, and is defined by its location at the rear of the property and other distinct and defining characteristics.  It has a flat roof with square, vertical sides usually built in brickwork to match the house or tile-hung or covered in cladding.  The back walls slope inwards at an angle of between 70 and 74 degrees to soften the look.  There are often windows added to the small dormers which project from the roof and create more space.  The flat roofs are built with a very slight degree of fall to the side to prevent rainwater from pooling on the top.

Why are homeowners choosing mansard roof options?

Loft conversions have never been as popular with the mediocre property market making moving house a less than inviting prospect and with the political and economic uncertainty caused by Brexit and then the Coronavirus.

Mansards are a great way of converting your loft especially when planning restrictions won’t allow you to have a rear dormer or a Velux loft conversion. Some people who have the choice will select a mansard over a dormer because they prefer the look – it’s not just all about the space.

How are the costs of a mansard loft conversion broken down?

The costs for your mansard loft conversion will be split between labour and materials.  You should expect to receive a very detailed description of the materials to be used.  There may be charges for machinery or plant hire such as scaffolding to access the external roof and a skip to dispose of any waste. You'll need to factor in the cost for reroofing the house as the structure of the roof is likely to change significantly. 

Are there any other cost factors?

You should factor in professional fees so the architect’s drawings and plans which will average in the region of £1,000 and also the application for planning permission which is a charge of just under £200 payable to the local council. An additional long term factor is estimated longevity and making sure you are aware of the potential roof repair costs should anything become damaged in the future.

In addition to the planning consent, your mansard loft conversion will have to be inspected by the building regulations department of the local authority to ensure it is being constructed correctly and in accordance with building regulations.   This is to ensure it complies with rules pertaining to fire safety, access and floor strength amongst other things.  There is a charge for submitting your architect’s plans for inspection and also having on-site inspections and this usually runs into a few hundred pounds. 

You may need a Party Wall Agreement if your house adjoins other properties on one or both sides (two different agreements with each neighbour) and this can result in additional expenditure if they have to make any adjustments to their property or if any damage is caused.

Depending on your location, your loft may need to have a bat survey.  Bats are a protected species under the 1981 Wildlife and Countryside Act and you may need to handle any bats resident in your loft before you apply for planning permission.  Usually, the planning consent will require you to conserve or rehome the bat colony but there are specialist ecologists who can advise you how to manage this issue safely and effectively.  It doesn’t mean you can’t convert your loft but you may incur additional costs managing the resident bats to the satisfaction of the local council planning department.  This is usually only a problem in older or rural properties.

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Hi I’m Sean. If I’m not up on a roof, or writing about roofing – i’m probably sleeping!

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What are the advantages of a mansard loft conversion?

  • Mansards can be built in different shapes and sizes, from a small window through to full-width mansards which run right across the back of the property
  • A mansard is a great way to create floor space and headroom in a loft which is conservative in size – the bigger the mansard, the greater the headroom and floor space
  • Mansards can be a real design feature on a house as well as a great way to add space and height to a loft conversion.  They can be built in matching brickwork or tile hung and offer real design potential
  • You can add French windows to your mansard
  • Mansards work very well with Victorian properties and are a popular choice for Victorian terraces where rear extensions may be limited but, mansards can actually suit all styles and types of property
  • The external walls of a mansard can be exposed brickwork, rendered or clad in slate or tile – whatever best matches your property

What is the process of adding a mansard loft conversion?

The first part of the works will consist of installing a structural floor including steels and timber floor joists.  Once this is complete, the old roof at the rear of the property will be dismantled from the exterior accessed via scaffolding and removed so that the mansard can be fitted.  The walls are raised and the new roof constructed at an angle. If part of the plans, dormer-style windows are installed.  Interior walls are fitted along with insulation followed by the windows.  Electrics and plumbing are next followed by plastering and carpentry work such as skirting boards.  Usually, there is a building regulations inspection at this point before the final decoration can take place.  This is in addition to the stage inspections which will have occurred earlier on in the works.

It usually takes just a few days to install a mansard to an existing loft conversion depending on the size but an entire loft conversion will take between six to eight weeks on average.

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Frequently asked questions 

Do I need planning permission?

You will need planning permission for a mansard loft conversion due to the extensive changes to the roof shape and structure. Planning permission is not normally required for a standard loft conversion but because a mansard completely alters the roof structure, then you will need to appoint an architect to draw up plans and make the necessary application.

What is an L-shaped mansard loft conversion?

An L-shaped mansard conversion is used to really open up the space in the roof void and works particularly well if you are going for an open plan bedroom with en-suite bathroom.

Which is best, a mansard or a dormer loft conversion?

Not everyone has the choice depending on the style of property and in some circumstances, planning restrictions limit the available options if the property is located in a conservation area.  A dormer will offer the greatest exploitation of floor space and headroom but a mansard will look softer from the outside although it won’t achieve as much head height compared to a vertical dormer. Expect to pay more for a mansard conversion.

What types of houses are best for mansard conversions?

Mansard conversions are very common in London and other metropolitan areas and often can be seen across the backs of entire rows of terraced properties.  But you can actually fit a mansard conversion to almost every type of property  - detached, semi-detached, bungalows and chalets - although they are probably most popular on a Victorian terrace in an urban setting due to the limitations on ground floor extensions.

How long will the conversion take?

It will usually take around 6-8 weeks once you have obtained planning permission, the final length of time ultimately being dictated by the fittings you have in the loft conversion – a new kitchen or bathroom is going to take longer to finish than a new bedroom – and whether or not you encounter any unforeseen problems along the way.

Do mansard loft conversions add value to my home?

A survey conducted by the well-known building society, Nationwide, concluded that a loft conversion which featured an extra bedroom and en-suite bathroom added an average of 21% to the value of a property: other studies have found the figure to be even higher.  However, it’s not just the additional accommodation.  A mansard loft conversion offers some fabulous design options which can create a real feature to a more conventionally built and furnished family home.  Using the open-plan space creatively with the additional floor area and headroom can turn a loft conversion from a useful extra floor into a striking rooftop design statement, a great way to bring a real contemporary vibe to a more traditional or Victorian home – the best of both worlds.

What are the disadvantages of a mansard loft conversion?

A mansard loft conversion is the most disruptive to your home and its structure, so it involves the most amount of work and therefore cost.  The architectural input is crucial and the building works are more complicated and will take longer than other conversions.  And you will require planning permission whereas a more conventional and standard loft conversion does not always.  And this is despite the fact that the conversion is at the rear of the property and does not affect the front facade.

How can I save money on a mansard loft conversion?

That’s tricky as it is a pretty substantial building project but you can significantly impact on the cost if you choose to avoid putting certain rooms like a bathroom or kitchen into the new space – these are the most costly and a large proportion of the estimate will be spent on the internal fittings.  Another way to shave off some money is to use one contractor who can offer the entire package including the plans and drawings and all the necessary consents rather than appointing a separate architect.  There are roofing companies who will handle everything right from the initial concept through to the finished conversion, it’s just a question of comparing their estimate with the more fragmented approach of appointing individual builders and architects. 

How can I fund a mansard loft conversion?

Most people seek further finance from their mortgage lender who can offer you funds via a Home Improvement Loan which is like an extension to your mortgage.  This is available if you have enough equity in the property and can satisfy their affordability criteria for the extra monthly repayments. 

Where can I find a reliable and reputable roofing contractor?

If you can, go with a personal recommendation from a trusted friend or colleague.  If not, social media, a local community group, is a good source of recommendations if you are new to the area.  Most people are only too happy to give their opinion when asked and if the recommendations are local then you should be able to go and see some lofts which have already been converted as an example of their work.  Don’t cut corners though on your assessment process just because someone has been recommended to you.