dormer loft conversion construction

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How much does a Dormer Loft Conversion Cost?

A dormer loft conversion costs between £30,000 and £60,000 due to the structural alterations to the existing roof. You could even splash out and have an entire suite of rooms engineered off-site and lowered onto the house by a crane. This involves the complete removal and replacement of the existing roof. Expect this to cost between £50,000 and £70,000.

Last Updated: October 2022

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Loft conversions have always been popular as the structure is already there and the space redundant and unused. If you intend to undertake little structural work but just add another room in your roof without a dormer, then the cost of a dormer loft conversion will come in at somewhere between £10,000 and £20,000 depending on what you choose to create.

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With an uncertain property market and the likelihood of an economic recession looming, many people are choosing not to move house but to extend their existing property. Whether you have little outdoor space or either don’t want to build an extension to the rear or side of your house or, budgetary constraints won’t permit it, almost everyone can make use of their loft space.

Much of the final cost of dormer loft conversion will depend upon your roof structure and pitch, the design of the conversion and what use you want to make of the available space internally.

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What is a dormer loft conversion?

A dormer loft conversion uses the existing roof void to create accommodation with a structure that extends beyond the existing roof space by projecting vertically from the slope of the roof.  Internally, the dormer usually has a horizontal ceiling and vertical walls.

Why are homeowners choosing roof dormers over other options?

Dormer loft conversions not only create additional floor space but increase headroom so they are ideal for small roof voids where there is insufficient roof height and/or floor area to feasibly create another room or, the room would be very small.  So you can create more usable room and the additional height also gives more options when it comes to the location of the staircase.

How are dormer loft conversion costs broken down?

Costs are usually split on a quotation between labour and materials.  Even with a simple loft conversion, you will still need to reinforce the floor and add insulation to the new room plus relocate plumbing and wiring.  However, there will also be professional fees to take into account. Use our roof cost calculator to understand how much you may need to budget for your project.

A dormer roof conversion because of the structural changes will require an architect.  If your house is semi-detached or a terraced home, you will need to notify your neighbours of the work under the requirements set out in the Party Wall Act.  You will have to enter into a Party Wall Agreement with your neighbours and there is a cost associated with this. This requirement is usually handled by the architect or surveyor. 

Your loft conversion will require Building Regulations approval throughout the process of construction with a final sign off at the end of the works.  A building control inspector will check and inspect the work at various pre-determined stages and when the work is completed in line with Building Regulations’ standards, he will issue a completion certificate after his last inspection.  There will be a fee associated with this process. The parts of the Building Regulations the Inspector will be concerned with will probably be:-

  • Parts B and P – fire safety and electrical safety
  • Part L- thermal efficiency standards for the new habitable space
  • Park K – escape routes and safety measures for collisions and impacts
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Hi there, I'm Sean Freeman

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

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Other Dormer Loft Conversion Cost Factors

  •  You will need a staircase from the floor below to gain access and there can be significant remodelling and redecoration work to be done depending on where this is sited.  The best location is usually as an extension to the existing staircase
  • If you don’t opt for dormer windows or even roof lights then there will need to be some provision made for ventilation
  • Factor in costs for the additional lighting and heating requirements so you will need not only the actual light fittings and radiators but an extension to the existing services in your home up into the roof area which may require a boiler upgrade.  Well insulated accommodation may not put too much extra strain on the household heating system but if you are installing a bathroom in the roof space, then you may have to consider a new boiler
  • You will lose the roof space for your water tank and associated plumbing so the heating and hot water system may need to be replaced with a sealed system.  A switch to an unvented system will remove the need for a header tank
  • You could need repairs or some upgrading to your roof – many homeowners use this as an opportunity to undertake some roof refurbishments particularly if they are choosing a dormer windows conversion.  A traditional framed roof is the easiest and most suitable for loft conversions but the rafters might need to be reinforced or require additional support. Trussed roofs require greater structural adjustment which will impact significantly on the costs to repair the roof.

What is the process of adding a dormer?

What is the process of a dormer loft conversion?

Your chosen architect will need to do a full site survey and inspect the roof for suitability for conversion.  Your builder and architect liaise, with the architect producing the drawings and obtaining the necessary planning consents from which the builder can provide a schedule of works and a detailed cost estimate.  Once planning permission is obtained and the process of Building Control approval is in place, then work can begin. This is a typical schedule of works:-

  1. The loft needs to be cleared and any water tank removed and refitted elsewhere 
  2. Wiring is assessed and if necessary, re-routed
  3. New floor joists are fitted followed by any new wiring and pipework
  4. The floor is insulated and floorboards laid
  5. Roof rafters are reinforced and then the roof opened up to accommodate the new dormer
  6. The new staircase is installed
  7. The dormers are tiled and vents fitted and windows fitted in the dormers if required
  8. The roof is insulated
  9. Partition walls are erected if there is to be more than one room
  10. The electrics are upgraded before plasterboard and skirting board is fitted ready for plastering and decoration

Dormer Loft Conversion Cost FAQs

Is my loft suitable for a dormer conversion?

The measurement from the bottom of the ridge timber to the top of the ceiling joist should provide a minimum of 2.2 metres of usable space.  Anything less than this would not be considered suitable. Building Regulations do not impose a minimum ceiling height for what is deemed to be a habitable room so don’t assume this offers guidance when it is actually absent.  If there is head height of less than 2.2 metres then there are still options for conversion but these will be much more costly. These include raising the roof or lowering the ceiling height in the room below. Raising the roof will of course add to your dormer loft conversion costs. 

Is my roof pitch right for a loft conversion?

The higher the angle of the roof pitch, the more central head height you will have.  If you then use a dormer roof conversion, you can increase both the usable floor area and the potential for more headroom. The pitch of the roof may affect your dormer loft conversion cost.

What does a dormer look like?

A dormer is the name of a roof structure which extends vertically beyond the plane of an existing roof pitch.  Dormers are used to create more usable space when converting a loft and also to create larger window features than may be achieved with standard roof lights or Velux windows.

What are the different types of dormer?

The two main types of dormer are a gable-fronted dormer also called a gabled dormer and, a Hip roof dormer.  A gabled dormer is the most common type of dormer and is a simple pitched roof of two sloping planes supported by the frame which rises vertically to form a triangular section below the roofline.  These are sometimes known anecdotally as dog house dormers. A Hip roof dormer is a roof which slopes upward on the three sides of its structure so there are no vertical ends. The point at which the three sloping sides of the roof meet is called a hip, hence the name.  The phrase, hip bevel, refers to the degree of the slope.

What is a Mansard loft conversion?

A Mansard loft conversion is normally situated to the rear of the property and has a flat roof with the back wall sloping inwards at an angle of 72 degrees.  A Mansard is a type of dormer roof conversion which offers maximum roof conversion space because it protects the highest available headspace and thus gives a greater usable floor area.

How much space does a dormer add?

As an average, a dormer loft conversion can add between 200-250 square feet of living space to your home although much will depend on the size and shape of the original loft and how you convert the interior.  With a dormer, you can add height and width and imaginative interior design can make even a modest incremental increase in space go a long way.

Will a dormer loft conversion add value to my home?

Improving and upgrading the fabric of your home will always increase its value.  Effectively you are adding an additional room or maybe rooms. Some statistics would add as much as 20% for a further bedroom and ensuite bathroom but do be aware that your house will have a natural ceiling to its value however fabulous your loft conversion is.

How long does a dormer loft conversion take?

A rear dormer loft conversion will take around 4-5 weeks from start to finish and a gable end rear conversion a little longer so between 5 and 6 weeks.  The only things which can delay the works are unexpected weather conditions and unforeseen problems which were not apparent at the time of the survey.

Can I convert my loft in the winter or should I wait until the summer months?

Most building companies who specialise in loft conversions have a weatherproofing system which will protect your home year-round.  The British weather is unpredictable and the warmer months, particularly August, can experience heavy rainfall levels. The main difficulty with the winter months is working at height in windy conditions and the shorter daylight hours which require the installation of external artificial lighting to avoid shorter working days.  Generally, most people opt for the summer months unless there is a particular overriding reason why the work needs to be carried out at another time of year, perhaps to link in with other projects.

Do I need planning permission?

Unlike new build extensions, most loft conversions fall within what is described as Permitted Development (PD) and a straightforward loft conversion will not normally need planning permission.  However, a dormer roof conversion extends the roof beyond its existing size and limits. 

Roof dormers can be installed under PD but only if they do not sit forward on the roof plane on the principal elevation.  They also cannot be higher than the highest point on the existing roof. Extending or altering the roof beyond its original size and effectively going outside PD so altering the roof height or shape, will require planning permission.  This will be issued with certain restrictions on the size of the conversion. If your home is a listed building or in a conservation area, then you will also need to apply for Planning Permission even if your roof dormer falls within PD.

What is the Party Wall Act?

If your home is semi-detached or within a terraced row then your dormer loft conversion will fall within the auspices of the Party Wall Act 1996.  The Party Wall Act was brought into force to regulate the situation when one party is working on a shared wall or boundary and to avoid disputes arising between neighbours.  Some of your works will impact on your neighbour/s so the Party Wall Act defines how this process should be managed so that there is co-operation between the parties and your neighbour’s rights are also protected.  If you fail to follow the legislative requirements, then your neighbour could obtain an injunction to stop the works from progressing. Usually, your architect or surveyor will manage this process as part of the regulatory compliance alongside planning permission and building regulations.

Will the loft conversion carry any guarantees for the work?

Most reputable companies will offer a structural guarantee on their loft conversions usually of around ten years plus a 12-month maintenance guarantee.

We hope that now you have a better understanding of the cost of a dormer conversion.


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