Private House - Holland Park :  Terrace by New Images Ltd

Everything you need to know to transform your semi

Johannes van Graan Johannes van Graan
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When it comes to the backbone of Britain’s housing stock, the semi-detached home is undoubtedly a favourable candidate, which forms a quintessential part of the streetscapes in most villages, towns and cities. 

Known affectionately as ‘semis’, these little extensions come in many guises – from the charming bay windows that so regularly adorn Victorian semi-detached properties, to the strong gables and rendered mock timber-frame façades that remind us of the golden age of architecture of the 1930s. Even the more modern brick-clad homes of the mid to late 20th century bring their fair share of semis to the table. 

But don’t think that remodelling and/or extending this type of property is a hassle-free home improvement project. Unlike the detached home, there is a significant factor which could impact (perhaps even put a pause on) your project ambitions: the neighbouring home with which the property shares a party wall.

So, if you’re considering overhauling a semi-detached property, what are the challenges and considerations that could be facing you?

1. Addressing the existing floor plan

The starting point is undoubtedly the existing layout of your property. Ask yourself the following questions to help identify your needs for the space:  

• Does it work for you and your family? 

• Do you make use of every room, or are there any areas which represent dead space? 

• Are there walls that can be opened up to create more usable rooms?

More often than not, a more efficient use of the existing space can be a better option than adding more floor area. So rather give thought to how you’re using your current space. Cutting circulation space down to a minimum could make a huge difference, like opting for an open-floor layout.

2. Extending a semi-detached home

Wandsworth Extension:   by Model Projects Ltd
Model Projects Ltd

Wandsworth Extension

Model Projects Ltd

Extending to the side or rear of a semi-detached home can be a great way to add more floor space, plus make the most of your garden too. Extending a semi presents fewer hassles, especially in terms of planning, than extending a terrace with immediate neighbours on either side. And there are also a lot less constraints when it comes to the build. 

A rear extension is a great opportunity for more possibilities, and to do something a little different as well. A lot of homes who opt for this type of build have retained their traditional street-facing façades (often to appease planners), with a more modern extension added to the rear.

3. Potential design issues with extensions

Single Storey Extension and Loft Conversion, Lance Rd: modern Houses by London Building Renovation
London Building Renovation

Single Storey Extension and Loft Conversion, Lance Rd

London Building Renovation

Overlooking can be a big issue for the semi-detached home, and sometimes adding glazing to the side elevations of a new extension may not be an option. In these cases, a potential solution could be to maximise glazing to the rear wall, paired with roof lights, lanterns and other forms of glazing to the roof. 

A new extension will result in a deeper floor plan, which creates central rooms that are devoid of natural light. You could consider transforming such rooms into open-plan layouts with the new extension to address this issue. 

What’s more, a new extension or exterior facelift may have to mimic the neighbouring home too, where the planners might require you to match certain materials, like the roof tiles or slates to the house next door. 

Of course it goes without saying that getting a professional architect or designer with experience with this type of project on board will only serve in your best interests.

4. Possible build issues with extensions

Private House - Holland Park :  Terrace by New Images Ltd
New Images Ltd

Private House—Holland Park

New Images Ltd

The building part of such a project can also definitely add its own share of challenges. For example, access to the rear side of the house might be limited, or you could face possible drainage problems, which typically runs to the side of the house. 

Bear in mind that an extension is not the go-to solution for every semi-detached home, particularly if the rear garden is small. In a lot of cases, planners require that a minimum of 50% of the garden remains.

5. Staying on good terms with those neighbours

• If this type of project is in your near future, be sure to speak with the neighbours before applying for planning permission (or, if the work falls under Permitted Development, before the plans are finalised). 

• Allow neighbours to have an input (even if you do not take their ideas on board) as it will put them at ease. At the very least, let them air their concerns. 

• Ask your architect or architectural designer to provide 3D visuals to demonstrate shadow and light, so you can show the neighbours how this may affect their property. 

• Choose a builder or contractor who will be respectful of your neighbours. 

homify hint: In your quest to find the perfect person for the job, be sure to follow up on as many references as possible.

6. The Party Wall Act

Marlborough Road - Extension to Single Room: modern Living room by Haydn Bennett Chartered Architect
Haydn Bennett Chartered Architect

Marlborough Road—Extension to Single Room

Haydn Bennett Chartered Architect

To keep it simple, the Party Wall Act was introduced to prevent building work undermining the structural integrity of shared party walls and neighbouring properties; helping to prevent disputes which could arise, and helping with possible solutions if and where they do. The following works are generally included: 

• Loft conversions (which involve structural work to the party wall); 

• Building a new wall (for an extension, for instance) up to or on the boundary; 

• Most excavation work, including foundations for extensions and basements.

It is recommended to appoint a surveyor to prepare a Party Wall Award. They will document how and when the work can be carried out, for instance, stating that building will not take place before 8am or after 6pm. More importantly, however, they will survey the neighbouring house (noting any existing cracks, for instance) prior to the project going on site, preparing a report on the condition of the property.

After the project has been completed, the surveyor will return to ensure that no damage has been caused to neighbouring houses. 

Remember that this can also protect you from any possible bogus claims.

7. Adding a new basement to a semi-detached home

  by Uma Casa Portuguesa
Uma Casa Portuguesa

Uma Casa Portuguesa—Ferraria de São João (Before)

Uma Casa Portuguesa

Converting an existing cellar or adding a new basement are also very popular ways in which homeowners conjure up new floor space, especially in urban areas where space is at a premium and the garden is limited. These types of projects tend to be financially viable in high-value areas such as the south-east and London. 

However, addressing your neighbours’ concerns is (again) key. You need to explain to them that you are making use of a seasoned and professional architect and contractor, that the plans have been passed by Building Control, that there is a Party Wall Award, etc.  

It is also worth remembering that building a basement could help your neighbour. For example, if you’re building up to the party wall, and your neighbour wants to add a basement themselves, you’ve already created a ready-made wall to one side, saving them money.

8. What will it cost?

Redesdaale Street Chelsea Basement Development Rooflight:  Windows  by Shape Architecture
Shape Architecture

Redesdaale Street Chelsea Basement Development Rooflight

Shape Architecture

A basic refurb (including basic wiring and decoration) will cost in the region of £500—£1,000 m². Adding an extension will cost around £1,500—£2,500 m², while basements are regularly priced in the area of £2,500—£3,500 m². 

Please remember, however, that there are classic elements of a project which could blow your budget, such as an ultra high-end kitchen. And never forget to factor in VAT with your costs. 

Check out these: 18 home improving extensions you can actually afford.

What are your plans for an extension? Tell us all about it...
Whitton Drive:  Terrace house by GK Architects Ltd

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