Loft Conversion Building Regulations: A Simple Guide

Loft Conversions 11 February 2022

A friend just called me to say: ‘I’ve spent 3 hours scouring the internet to try and understand Loft Conversion Building regulations and am more confused than when I started!’

The first question I asked was ‘why?’

The reason is that there are hundreds if not thousands of different building regulations guides online with lots of conflicting and sometimes technical information.

So my challenge was to prepare this guide, send it to that same friend, who would read it and most importantly understand the entire process by the end.

The guide below explains the entire process from design, planning, all the way through to construction.

Why are loft conversions so popular?

This simple guide sets out all the stages you should consider before starting your loft conversion. We cover everything from design and planning permission to loft conversion building regulations and lighting.

Loft conversions are one of the most popular types of home extension in the UK. In addition to the considerable space they can add to a property, they can often also be built on houses without any need for planning permission.

It is estimated that if you add a one-bedroom and bathroom loft extension to your three-bedroom house, you could add around 20% to the value of a home. If you calculate the additional value to the average priced home standing at £230,000 (ONS, 2019), you could add up to £45,000 to the asking price. If you live in London or another UK city, it is likely to add more than this, as average house prices are far higher.

Overview of the loft conversion building regs process

The loft conversion process can be separated into 4 simple phases:

1. Design

Obviously this is the most important stage as it’s important to design something which looks spectacular. Usually, an architect would discuss your ideas and then go and design a full set of plans, taking into account all your ideas and a few creative ones of their own.

2. Planning

The second stage is to obtain the necessary consent. When it comes to lofts, that will either be applying for planning permission or if your loft design is considered to be Permitted Development your architect would apply for a Certificate of Lawful Development instead.

3. Building Regulations

Congratulations! Your loft conversion has now been approved. Now comes the more technical part of the process. Your architect will need to prepare a full set of detailed drawings showing things like insulation, beams and foundations. Once those drawings have been prepared they would be sent to an engineer who will prepare a set of structural drawings and calculations. Once those are ready, you will have sufficient details to submit your application to Building Control for approval. At this stage, you can give your plans to a builder who should then be able to give you a fixed quote.

4. Construction

Once Building Control approval has been granted, your chosen builder will begin the construction process which could take between 2 and 3 months for most loft extensions depending on the complexity of the design.

Types of loft conversions

There are many different types of loft conversion, but there are three which seem to be more popular than others.

Dormer window

3D model of a loft conversion with a rear dormer window

Dormer windows have a flat roof and vertical walls so are a great way to create more space in your loft. This is the most common type of loft extension and can be constructed in a number of different materials such as clay hung tiles render or even Zinc (which is our favourite as it gives a really modern twist to the build).

Hip to gable

3D model of a hip to gable loft conversion

Most semi-detached and detached houses benefit from hipped roofs which leaves little space for a dormer window to fit comfortably within the roof slope. A hip to gable loft conversion dramatically increases the usable space within your loft and can also be combined with a rear dormer window to create a fantastic new room.


3D model of a Velux window loft conversion

Velux loft conversions are a good idea as they are more cost effective when compared to dormer windows and gable extensions. Velux are a company who are the leaders in roof windows and they sit completely flush with the main roof. There is limited building work involved and this therefore keeps the costs down whilst also creating usable space.

Can my loft be converted?

Whether your loft can be converted depends on several different factors.

Loft height

Although the loft conversion building regulations do not include minimum ceiling heights for habitable rooms, there should be at least 2.2m between the bottom of the roof timber and the top of the ceiling joists below to be considered usable space. It is worth bearing in mind that there needs to be a minimum of 2m of height above the stairs.

If there is less than 2.2m head height when you measure, there are two solutions.

Solution 1: Raise the ridge height

Aside from the cost of raising the roof, the main obstacle here is whether your Local Planning Authority would grant planning permission. As a general guide, if you live in a road characterised by houses of the same eave and ridge heights it may be challenging to raise the ridge above the existing level, as it may be considered out of character.

Solution 2: Lower the ceiling in the room below

There is a lot of building work associated with such a change, e.g. the new floor joists will need to be supported by a plate bolted into the wall below with shield anchors or raw bolts. You also need to consider that whatever space you gain in the loft is then lost downstairs.

Roof pitch

Some roof pitches are shallow, which means the internal head height is low.  Consequently, you have less usable space. The steeper the angle, the higher the head height and the more space there is for you to enjoy.

Roof structure

Traditional frame type roof construction lends itself more easily to a loft conversion, as the space can easily be opened up with minimal cost. That being said, the rafters may need to be strengthened, but a structural engineer would be the best person to advise on this level of detail.

Which types of loft conversions need planning?

The majority of houses in the UK benefit from Permitted Development Rights. This means there are specific extensions you can build without the need to obtain planning permission.

However, there are several criteria the design needs to adhere to. Check out the advice offered by the Planning Portal who provide beneficial guidance on various types of extension.

Four of the most important criteria are:

  • A volume allowance of 40 cubic metres additional roof space for terraced houses
  • A volume allowance of 50 cubic metres additional roof space for detached and semi-detached houses
  • No extension beyond the plane of the existing roof slope of the principal elevation that fronts the highway
  • No extension to be higher than the highest part of the roof.

Do I need planning permission for loft conversion?

If you live in a flat, conservation area or listed building, you will not benefit from Permitted Development rights. Therefore, you would be required to apply for planning permission if making external changes to your property when converting your loft.

Each borough in London will have its own particular set of planning rules when it comes to loft extension, but as a general rule of thumb the majority of policies will state that dormers should:

  • Be half the depth and height of the roof slope
  • Be subordinate features of the original roof
  • Not wrap around the hips or rise above the ridge
  • Allow adequate roof space above and below the dormer.

Who do you have to notify for loft conversion building regulations?

When it comes to obtaining the relevant consents you are not required to let your neighbours know of your plans to build a loft conversion, but it’s always nice to be neighbourly so it can’t do any harm to let them know about your plans.

The Permitted Development Route: If your architect or planning consultant suggests this is the best route once your certificate of lawful development has been submitted, your local planning authority will not consult your neighbours as the works don’t require planning permission. Your application will however be visible on their planning applications website.

Planning Permission Route: If you go down the planning permission route your local planning authority will notify your immediate neighbours in writing with the details of your application. Neighbours will then usually have a period of 21 days by which to comment.

Who assesses whether your loft complies with the relevant regulations?

Once your planning application or certificate of lawful development has been submitted, it will be passed to a planning officer who works for your local authority. They will either use their local planning policies and guidance to assess your application or the General Permitted Development Order if your application is considered to be Permitted Development.

Loft Conversion Building Regulations

If you are converting your loft, you will need building regulations approval. If you own a terraced or semi-detached house, you will also need to notify your neighbours before you start works if they are covered by the Party Wall Act.

Building regulations are designed to confirm that your conversion is structurally sound, meets all the relevant fire regulations, and is well insulated.

The type of building regulations you need to adhere to will largely depend on the type of loft conversion you are proposing. Some standard requirements are highlighted below:

  • New stairs will be required to provide an escape route in the event of a fire
  • Floor and beams will support the increased weight and new joists will likely be required
  • Sound insulation will ensure the noise transfer between rooms should be kept to a minimum
  • Fire safety should be assured with the use of fire-resistant doors, and mains-powered smoke alarms will need to be fitted.

This is not a definitive list but rather a brief guide. For a detailed assessment of loft conversion building regulations, you can call one of our architects any time for a free consultation.

How Much Does a Loft Conversion Cost?

The cost of your new loft conversion will vary and depends on several factors including the height of your ridge, the pitch, whether a new staircase can be easily accommodated, and the type of finish you would like.

The three examples below are the most popular options selected by clients choosing a loft extension.

Loft Conversion Room

A loft conversion room involves minimal external changes, as the majority of things changed are internal to the property.

The main changes required include:

  • Reinforce the existing loft floor
  • Front and rear skylights
  • New staircase from first floor
  • Lighting, heating and plumbing
  • Fire safety measures.

Cost: This option would cost approximately £15,000 and is the cheapest and quickest option to build.

Hip to gable & dormer loft conversion

The hip to gable and dormer loft conversion is one of the most common types of loft conversion. This option has the potential to add a considerable amount of space to your loft. Depending on the height of your existing loft, most homes can comfortably accommodate a master bedroom and en-suite bathroom.

This type of conversion can typically be constructed through Permitted Development, thus eliminating some of the risk associated with applying for planning permission.

Cost: This type of loft extension will cost between £40,000 and £55,000 depending on the finish.

Change of the entire roof structure

In some instances, where you are linking a loft conversion to a two-storey side or rear extension, it could be feasible to rebuild the whole roof. Sometimes, this also allows you to change the pitch and ridge height to obtain even more space.

Cost: These loft extensions are the most expensive, would typically cost upwards of £60,000, and are only suitable for larger houses.

Do I need an architect for loft conversion building regulations?

There are two main options when it comes to designing a loft conversion.

Option 1: Architect

A creative architect will be well placed to take your design ideas and turn them into a genuinely bespoke room for you to enjoy. They can prepare various options and also 3D visuals to help you better imagine the space. They will also have a firm grasp of the multiple policies, obtain the relevant consents, and manage the process through building control.

Option 2: Loft Conversion Company

The advantage of a loft conversion company is that it’s a one-stop-shop which makes the process a bit smoother. That being said, it can be a good idea to separate the design and build because these companies will not employ architects, which means there won’t be as much scope to be creative and look at various options to suit your needs.

How to bring in lots of light

When designing a loft conversion it is essential to maximise the light to improve the sense of space. You have three main options.

Roof lights

Roof lights typically let in double the amount of light compared to a conventional vertical window and three times more daylight than dormer windows. They are also excellent for ventilation, as fresh air is drawn up through the house using natural buoyancy and pressure difference.

Dormer Windows

Dormer windows allow you to substantially increase the overall head height in your loft. The higher your existing loft, the bigger the space that can be created. As most dormer windows in the UK are built through Permitted Development (without the need for planning permission) they do not necessarily need to follow the Local Authority’s planning policies concerning window sizes. When it comes to Permitted Development loft conversions, there are no rules regarding how big the windows can be towards the rear, so one could feasibly make an entire wall of glass to maximise light if they wished.

Artificial light

As in any lighting scheme for the entire house, the best results come when different artificial light sources are combined:

  • Ambient light (substitute for daylight)
  • Tasking lighting (reading)
  • Accent lighting (to add atmosphere).

Heating a Loft Conversion

When building most extensions, the boiler will need to be upgraded.  The upgrade is necessary because of the increased heat load requirements. That being said, the loft space itself will require little extra capacity as it will be better insulated than the existing loft and can actually increase the house’s thermal efficiency.

If a bathroom is added, a new boiler may be required. In this case, it can make sense to switch to an unvented system that does not require header tanks and instead relies on mains pressure.

Insulating a loft extension

There are two main ways to insulate a loft conversion.

Cold roof Insulation

This method involves fitting insulation between the rafters and insulated plasterboard, which helps reduce thermal bridging. The advantage of this method is that it is cheaper to fit and can keep your house colder in summer.

Warm roof loft insulation

Instead of insulation being fixed in between the rafters, the insulation in this method is laid over the rafters. This involves covering capping, followed by fixing battens and tiles. This ensures the entire roof structure is well insulated, meaning it is more energy-efficient than a cold deck roof.

Adding a bathroom to a loft conversion

With the increased popularity of en-suites, it’s now common to put a bathroom in a loft conversion. The most efficient way to make the plumbing work is to site it as close to the existing waste and supply pipes as possible. This will make it both easier and cheaper to fit your bathroom. Here are some additional tips:

  • The shower should be placed at the highest point
  • A bath can go under the eaves
  • The void in stud walls can be used to conceal shower mixers
  • Wall-mounted sanitary wear frees up the room giving the illusion of more space
  • Effective lighting and the use of mirrors can make a small bathroom appear larger.


There’s no doubt that a loft conversion can add a great deal to your home. You spend a lot of time in your home, meaning that it’s incredibly important to ensure that the space you have available works as well as possible for you. Our team have a huge amount of experience of designing loft conversions and can offer you a lot of help and advice as and when you need it. Simply give us a call and we can get the ball rolling and create a loft that you are sure to be thrilled with.

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