Semi-detached House Extensions: Rules & Guidance | MATA Architects (2024)

Read an example case study for the ground floor and roof extension of a 1930s Semi-detached London house.

How far can I extend my semi-detached house and do you need planning permission?

You’ll need to start by assessing whether Permitted Development (PD) rights would apply to your property and proposed extension. If PD rights do apply, you will not need explicit planning permission from your local authority. Learn more here.

Under PD you may extend your house by 3m or 6m if your site does not fall on designated land. More on that later.
The question of how far you can extend your house is primarily dependant on its context – planning history, location, proximity to neighbours etc. You can make some basic assumptions on the maximum overall size of an extension based on PD rules however, more space at any cost is not always everything. However, there are often better ways to create the feeling of more space without adding the largest extension possible.

A different question to ask yourself might be, “how far do I need to extend my house to achieve my goals?”. This is better starting point for planning your extension.

Semi-detached houses can usually accommodate 2 storey extensions although they are not as common as single storey extensions. They are a riskier proposition at planning stage due to a greater impact on neighbours and add to the construction and structural complexity of the project.

Any extension at first floor will be subject to the ‘45 degree rule’. In principle any extension should not exceed a line taken at 45 degrees from the centre of the nearest ground floor window of a habitable room. This rule is used as a guide by planners to assess the acceptability of the massing of an extension on both plan and elevation. Read more about the 45 degree rule here.

There can be some advantages to considering a 2 storey extension over a single storey extension. The overall cost per square meter for your extension will usually end up lower the larger the extension is. This economy of scale can result in better value for money.

It may be possible to extend your property under permitted development (PD) and avoid the need for full planning permission. You will need to ensure that your home does not fall the criteria that restricts or removes permitted development rights altogether. This includes any property subject to a planning condition or article 4 directive (often the case if you are living within a Conservation Area). The materials used in a PD extension must also be similar to those of the original house which limits design.

In addition to location or type of property there are criteria that PD extensions must meet with regards to height, depth and width.

The key rules relevant to a single storey extension for a semi-detached house include:

• Only half of the area of the land around the ‘original house’ can be covered by an extension (including sheds and outbuildings).
• The extension cannot be built forward of the ‘principal elevation’ (front building line).
• Side extensions are permitted and to have a maximum height of 4m and a width no more than half that of the original house.
• Where the extension comes within 2m of the boundary the height of an extension cannot exceed 3m.
• A single storey rear extension cannot extend beyond the rear wall of the ‘original house’ by more than 3m (which excludes wrap around extensions in many cases).
• Where the property is not on Article 2(3) designated land (Conservation Area, AONB etc) the maximum depth extension increased to 6m. This approach also requires the relevant Local Planning Authority to be informed of the proposed works via a ‘Prior Approval’ notice before commencing construction.
• The extension cannot exceed 4m in height.

Note: The term ‘original house’ means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date).

A 2 Storey extension to a semi-detached house will also be allowed under Permitted Development if it meets the following key criteria (and does not have restricted PD rights due to location):

• The extension must not extend beyond the rear wall of the ‘original house’ by more than 3m or;
• Be within 7m of any boundary opposite the rear wall of the house.
• The roof pitch must match the existing house as far as is reasonably practicable.
• Any windows to upper floors located in a ‘side elevation must be obscured and non-opening (unless the openable part is more than 1.7m above the floor).
• Where the property is on Article 2(3) designated land (Conservation Area, AONB etc) extensions of more than one storey do not fall under permitted development.
• No balconies or verandas are covered under permitted development.

This is by no means an exhaustive list of criteria. You can read the full technical guidance on permitted development rights for homeowners here.

If you were to pursue the route of a Permitted Development extension, we would recommend that a Certificate of Lawfulness of Proposed Use or Development (CLOPUD) is sought from the local authority in order to create a formal record of the works and their legitimacy.

In our experience the cost of extending and refurbishing a semi-detached house in Greater London costs between £2,500 – £3,000 p/sqm.

A double storey extension would be a lower cost per sq.m due to economy of scale with a larger construction project (shared foundations, services etc).

Construction costs for house extensions can vary widely depending on a range of factors including:

• Spec of fixtures and finishes (Particularly the choice of glazing, kitchens, bathrooms, underfloor heating)
• Site constraints (access to site for construction, proximity to neighbours, space for site welfare etc)
• Below ground conditions (soil type, location of drainage and utilities)
• Type of construction system used (timber frame, concrete frame, steel frame etc)
• Type and size of extension (Single/2 storey, basem*nt, loft etc)
• External landscaping requirements
• Location (Construction in Greater London is generally higher than other areas of the UK)

These are very high-level cost estimates and are only appropriate for very early-stage budgeting. We have written more extensively about the costs involved in building a house extension here.

Redefine your spaces!

Semi-detached house extension ideas:

Opportunities for extending a semi-detached house include wraparound, single or multi-storey, courtyard extensions and also mirroring a double extension with your neighbour. See 15 house extension ideas here.