The Planning Portal Volume Calculator helps you work out the cubic volume of your roof space or extension. This measurement is essential when checking whether your project falls within Permitted Development Rights (PDR) or requires planning permission.
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A roof volume calculator helps you estimate the total cubic volume of your existing loft space. It measures the available space within your roof structure before any conversion work takes place.
This is important because certain loft conversions, including dormers and hip to gable designs, must stay within set volume limits to qualify for Permitted Development. For most houses, the Permitted Development rules allow:
If your loft extension exceeds these limits, you will need full planning permission.
If you are planning a loft conversion and want to know whether it can be built under Permitted Development, the roof volume calculator gives you a quick way to check.
It shows the starting size of your loft, the additional volume your design will add, and whether the total increase is likely to exceed PD limits. This helps you avoid unexpected planning requirements, refine your design early and keep costs down by staying within the permitted allowance.
To calculate your roof volume, you only need three measurements: the loft length, width and height from the joists to the ridge. These do not need to be precise; good estimates are enough. This gives you a clear idea of whether your design is close to the Permitted Development limit.
You need to calculate your roof volume to check whether your loft conversion can be built under Permitted Development (PD) or if it will require full planning permission. Roof volume is also useful during the early design stage to understand how much space your existing loft offers and whether your chosen conversion type is suitable for your property
Calculating roof volume is useful whenever you are exploring a loft conversion and want to understand how much space you can add under Permitted Development. It helps you check what is feasible, whether planning permission is likely and how different designs will affect your roof structure.
Dormer loft conversions often need a volume check because they add a noticeable amount of new roof space. Hip to gable extensions also increase cubic volume by replacing the sloping hip with a vertical gable, so the calculator helps you see if the extension fits within your allowance. L-shaped dormers can quickly push terraced homes close to the PD limit, making accurate calculations essential.
If you want to avoid full planning permission, knowing your starting volume early reduces the risk of costly redesigns. The calculator is also helpful for homes with limited initial loft space or for phased projects where multiple extensions might cumulatively exceed the PD threshold.
Roof lift loft conversions almost always exceed PD allowances because they raise the ridge height. If you are planning a piggyback conversion, you will need full planning permission, regardless of volume. However, calculating volume can still help you understand your existing loft size and compare alternative options
Permitted Development sets a maximum amount of roof space you can add before planning permission is required. These limits apply to your entire roof, which means any previous dormers, gable extensions or loft alterations count towards the total.
For most homes in England, the limits are:
If your proposed loft conversion pushes you over these limits, or if you already have an older roof extension that uses part of the allowance, you will need full planning permission.
Some properties do not have Permitted Development rights at all. These include flats, maisonettes and listed buildings. Homes in conservation areas or other protected designations may also have reduced rights or no rights, meaning planning permission is almost always required.
Before starting any design work, it is a good idea to check your property’s status with your local planning authority.
To use the roof volume calculator, you only need three basic measurements: length, width and height of your loft space. You can take these yourself with a tape measure and a notepad. Measurements do not need to be perfect, but try to be as accurate as you can.
Always put safety first. Only enter your loft if it is safe to do so and there is secure access and lighting.
The length is the distance from the front of the house to the back.
The width is the span across the house from side to side.
The height is the vertical distance from the loft floor to the highest point of the roof.
If you already have drawings for a planned dormer, hip to gable or roof lift, you can:
If you do not have drawings yet, use your best estimate to see how changes might affect your roof volume.
Once you have the loft length, width and height you are ready to enter them into the roof volume calculator. The tool will then estimate your existing roof volume, the new roof volume and how much extra space your loft conversion could create, as well as whether it is likely to stay within Permitted Development limits.
Different loft conversions change your roof volume in different ways. The roof volume calculator uses simple geometric formulas to estimate these changes, helping you understand how much additional cubic space each design will add. Below are the most common loft types, how their volume is calculated and examples to make the process easier to follow.
A standard pitched roof forms a simple triangular shape. To estimate its volume, the calculator uses the formula:
½ × width × height × length
This gives an approximate cubic volume from eaves to ridge. Most homes start with a volume of around 25 to 40 cubic metres, depending on roof height and width.
Example: If your loft is 6m wide, 2.5m high and 8m long:
½ × 6 × 2.5 × 8 = 60 cubic metres
This is your baseline volume before any extension is added.
A hip to gable conversion removes the sloping hip and replaces it with a full-height vertical gable wall. This increases usable floor space and adds extra cubic volume to your roof.
Typical increase: 10 to 15 cubic metres (though this varies with roof width and pitch). When combined with a rear dormer, the total increase is often much larger and must be checked carefully against PD limits.
Example: If your existing loft volume is 35m³ and your hip to gable adds 12m³: 35 + 12 = 47m³ total.
A terraced home would exceed the 40m³ PD limit, while a semi-detached home would remain under the 50m³ allowance.
A rear dormer extends the roof vertically, creating a box-shaped addition that significantly increases internal headroom and usable space. Typical increase: 15 to 30 cubic metres and volume added depends on:
Because dormers are one of the largest volume increases, they are a key element to check when assessing PD compliance.
Example: A dormer measuring 5m wide, 2.2m high and 2m deep adds:
5 × 2.2 × 2 = 22 cubic metres.
This alone may take a terraced home close to or over its PD limit.
An L-shaped dormer uses two connecting dormers to extend both the main roof and the rear outrigger (common on Victorian terraces).
Typical increase: 25 to 35 cubic metresThis is one of the fastest ways to use up your entire PD allowance. Terraced properties in particular must check volume very carefully, as these additions can exceed the 40m³ limit even without other extensions.
Example: Existing loft volume: 28m³ + L-shaped dormer addition: 30m³ = Total volume: 58m³
This would exceed PD limits for all home types.
A mansard loft conversion changes the whole roof shape by replacing the slopes with a near-vertical wall and a shallow top slope. This creates the largest volume increase of any loft type. Mansards almost always exceed Permitted Development and require full planning permission. Typical increase: Too large to qualify under PD. Often similar to adding an entire new floor.
Example: A mansard roof might increase overall roof volume by 40 to 60 cubic metres or more, depending on the property.
Roof volume is one of the key factors that determines whether your loft conversion can be built under Permitted Development or will require full planning permission. Permitted Development rules set strict limits on how much additional cubic space you can add to your roof.
If your proposed design exceeds these limits, planning permission becomes compulsory. If your design exceeds these limits, planning permission is required.
Knowing your existing roof volume and how much extra space your conversion will add helps you choose the right loft type, avoid redesigns and ensure your plans meet structural and planning rules before work begins.
Homes in protected areas are subject to stricter planning controls, and in many cases, Permitted Development rights do not apply. This means roof volume limits are either reduced or removed entirely, requiring full planning permission for most loft conversions.
If your home is in a Conservation Area, Area of Outstanding Natural Beauty (AONB), National Park, World Heritage Site or the Norfolk or Suffolk Broads, local authorities place tighter restrictions on how much you can alter the roof structure. Even small increases in roof volume, such as a modest dormer or hip to gable extension, may be refused under Permitted Development rules and must instead go through a full planning application.
Unlike standard properties where volume limits are clearly set at 40m³ or 50m³, special-designation homes are assessed on a case-by-case basis. Planners will assess the visual impact of the extension, its effect on the area's character, and whether it preserves the appearance of neighbouring buildings. In many cases, roof lift conversions, large dormers and L-shaped designs are unsuitable for protected areas because they significantly alter the roofline.
If your property is within one of these areas, always assume that planning permission will be required, regardless of volume, and that the design may need to be more conservative to gain approval.
Our experts break down common questions about roof volume calculators to give you better insight into how to proceed with your loft conversion.
You only need a volume calculation if you want to build under Permitted Development (PD). PD rules limit the amount of roof space you can add, so calculating the volume helps you verify that your design remains within those limits. If you already know your conversion will require full planning permission (such as a piggyback or mansard roof), a volume calculation is helpful but not essential.
Yes. A hip to gable conversion replaces the sloping hip with a full-height gable wall, increasing the roof's cubic volume. This increase usually ranges from 10 to 15 cubic metres, depending on the roof width and pitch. It must be included when assessing whether the overall enlargement stays within PD limits.
Yes. Any dormer that increases internal space adds to the roof’s total cubic volume. A full-width rear dormer typically adds 15 to 30 cubic metres, and an L-shaped dormer can add 25 to 35 cubic metres. All of this counts towards your PD allowance, which is why volume checks are important before designing a dormer.
Yes. A volume calculation is not required for most planning applications because the council assesses the design, size and appearance directly from the drawings. Volume calculations are primarily needed for Permitted Development checks, not for full planning submissions. However, including your volume figures in the design process can help ensure accuracy and avoid confusion later.
