Designing and Building Your Dream Home Extension: A Complete Guide to Planning, Designing and Costs

House extensions enable homeowners to expand their homes while adding value, and rate as one of the best investments you can make. 

A study commissioned by Nationwide found that the value of extended 3-bed semi-detached houses increases by around 23% on average. Given that the average home in the UK is worth around £270,000, a 23% boost to value equates to just over £60,000.

Of course, house extensions also enable homeowners to stay where they are while boosting the size and scope of their home, which is perfect for making the most of extendable houses or plots or for increasing the size of a home ahead of starting a family, etc. 

This is the ultimate guide to house extensions, covering options, Planning Permission and development rights, costs, processes, and more. 

The benefits of a house extension

For many, the benefits of a house extension speak for themselves. Whether you want more space to make way for new family members or simply want to invest in your financial future, extending your existing property is rock-solid. 

Here are the benefits of a house extension: 

1: Add new space

Here’s an obvious one: extensions add new space to a home. This is unsurprisingly the main motivator for why people consider extensions. You might need new space if you’re planning a family or if you want to make your living space more comfortable. 

Aside from making space for kids, extending a home unlocks opportunities to build home offices, workshops, cinema rooms, games rooms and more. Another popular option is to extend kitchens outwards, creating an attractive and modern open-plan living and dining area.

Here are some ideas of how you can use a new extended space: 

• Bedroom: You can create new bedrooms if extending above a garage or other flat roof. Double-storey extensions are another option for creating new space across both the ground floor and first floor. Bedrooms can be built with en-suites, enabling homeowners to shuffle their current rooms to make way for new family members. You could even add a walk-in wardrobe or other luxury additions like a wet room! 
• Dining room or kitchen-diner: Open plan living areas are incredibly popular. Extending downstairs areas creates a unique opportunity to fuse kitchens, diners and living rooms, creating gorgeous open-plan spaces. Open plan living is hot right now.
• Games or playroom: Games or playrooms don’t need to be very large and are often combined with other rooms when extending either down or upstairs. Of course, games rooms aren’t just for young kids - they’re perfect for big kids and adult kids too!
• Living room: Ground floor conversions create new space for living rooms, kitchens and fusions between the two. Check out  Good Housekeeping’s list of living room ideas for inspiration. 

2: Cheaper than easier than moving

Tying in with the above, extending is cheaper and easier than moving house. Moving is a hassle, but it’s also expensive and stressful, involving some of the following:

• Moving all your possessions.
• Possibly changing jobs.
• Possibly changing schools.
• Researching a new area.
• Solicitor fees.
• Stamp duty.
• Switching energy and insurance.
• Uncertainty about neighbours.

Sure, an extension involves some level of disruption too, but nowhere near as much as moving! 

Extending allows you to stay in the same place while rejuvenating your home, which brings us along to the next benefit:

3: Rejuvenates your home

If you’ve lived in your home for many years, extending is a great way to breathe new life into a space you’re already familiar with. Adding more space unlocks new ideas, whether creating a large open-plan kitchen and living area, building a workshop or home office, or adding bedrooms to accommodate new family members. 

Many people choose to renovate their homes at the same time as extending. This makes sense, as rooms will need to be stripped and gutted to a degree already. Extensions are a perfect choice if you’re looking for an excuse to redesign your home.

4: Maximises usage of space

Many homeowners in the UK don’t make use of the space available on their land. While gardens are important, excessively large gardens are time-consuming to maintain and can be built on. An extension isn’t likely to take up that much garden space either - even extending by 3m to 4m adds a vast amount of space to a kitchen, dining room or living room. 

Garages, corridors and other unused space can also be converted, modified or extended. 

For example, extending on top of garages and converting the garage portion into a downstairs living space adds multiple rooms to a house for the cost of just one extension. Extending converts disused space into valuable living areas.

5: Adds value

We’ll dive into this in more detail shortly. House extensions rate as a superb investment and are nearly always profitable over time. The average extension boosts property value by some 20% or more in areas where space is already at a premium. 

Wherever you live in the UK, extensions are economically wise. For example, extensions in the north of England won’t boost property value by the same margin as London and the South East, but they’ll also be cheaper to build. Moreover, as house prices increase rapidly outside of London, larger homes are likely to top people’s lists as desirable places to live. 

Building an extension is an investment that goes beyond providing additional space to your home. 

Will an extension increase the value of my home? 

As noted, house extensions boost the value of a home by around 15% to 25% on average. 

In 2022, the average house price in the UK was £278,000, according to the ONS. 20% of this is around £55,000.

The ONS has an interactive tool for checking the potential value an extension will add to your home. 

• The average 25m2 extension in England and Wales is worth around £60,000.
• Larger 35m2 extensions are worth around £85,000.
• Smaller 15m2 extensions are worth around £35,000.

This varies throughout the UK. For example, a medium house extension in Kensington and Chelsea is worth around £450,000! 

As an investment, Zopa found that extensions produce a return on investment (ROI) of around 71%. This means that an average- cost extension of around £19,750 returns profits of around £14,000. 

In short, extensions are probably one of the best investments you can make aside from purchasing a property. In some parts of the country, even small expansions can add massive value to a home. 

There’s no doubt that extensions are an excellent investment for homes and their homeowner’s economic futures. Adding more space while boosting a home’s value enables homeowners to enjoy their home more, and sell it for more in the future. House extensions are profitable in the vast majority of cases, returning healthy profits on the initial outlay. 

Is my property suitable for a house extension?

UK planning and development laws have become increasingly relaxed in recent years, granting homeowners the ability to extend with minimal red tape. Permitted Development Rights (PDRs) are more liberal than ever, even allowing homeowners to build larger double-storey extensions without Planning Permission. 

However, not all properties are immediately eligible for extensions. Here are 5 things to bear in mind if you’re considering a house extension: 

Q1: What are the planning constraints in your area?

Permitted Development Rights (PDRs) are a legal framework that provides homeowners with certain rights to alter their homes without Planning Permission. PDRs apply to England and Wales - Scotland has its own similar rules. 

We’ll cover PDRs in detail shortly, but it’s sensible first to consider whether you’ll need to apply for Planning Permission and/or Party Wall Agreements. If you live in any of the following areas, it’s best to prepare yourself for Planning Permission rather than assuming PDRs apply:

• Conservation areas.
• National Parks.
• Areas of Outstanding Natural Beauty.
• Norfolk and Suffolk Broads.
• World Heritage Sights.
• Conservation areas and Sites of Specific Scientific Interest (SSSIs)

If you live in one of these areas, you’ll probably have to navigate additional restrictions, such as constructing the extension from the same materials as the rest of the home. 

If you live in a Listed Building, you’ll also need Listed Building Consent before development - failure to obtain it is a criminal offence. Moreover, if you live in a historical or period area, there’s a possibility that the Local Planning Authority has placed an Article 4 direction on the area, which limits PDRs. 

Q2: Do I have enough room?

To extend, you’ll need sufficient room on your land. Gardens are the most popular space for extending, but extending upwards from garages or other flat roofs is possible. Be aware that extending to the front of your house will be tricky without Planning Permission. PDRs are limited by how much room you have to extend. 

For example, if your extension comes too close to the neighbour’s property, you’re more likely to need Planning Permission. The more room you have, the more likely you’ll be able to extend smoothly. 

That isn’t to say that small, tight extensions are out of the question, but be prepared to go through Planning Permission. 

While Planning Permission is often made out to be an awful, long-winded process, this is rarely the case these days. Most applications are granted or refused in 8 weeks. Providing sufficient plans, technical drawings and supporting evidence to the Planning Authority will greatly increase the odds of success, which Fast Plans will help you with. 

Q3: Will you alter the walls or foundations?

If you’re extending outwards into a garden or backyard, you probably won’t need to alter existing foundations unless you plan to knock internal walls down or build another storey. 

However, if you want to create an open-plan space, be aware that it won’t always be possible to knock down internal walls without adding additional reinforcements. 

If you want to build a new storey, then your existing foundations may need to be strengthened to ensure they can take the weight of the new floor. 

Q4: Will my house extension comply with Building Regulations?

All house extensions must comply with Building Regulations, which specify a pretty exhaustive list of rules for everything from insulation to fire safety and ventilation. 

Walls, doors and floors may need installing or upgrading, especially when the extension creates an open plan space (as fire spreads easily through an open space). 

Reputable contractors are experienced in negotiating Building Regulations, and planning services such as Fast Plans will help you submit plans to Building Control for approval. 

Q5: Is my house too old?

Older period homes built before the 1930s are unlikely to support most types of extensions ‘out of the box’ (many homes that are old will be listed too). If your home is listed, you’ll need Listed Building Consent before starting. 

It’s still possible to extend older homes, but upgrades to the walls, foundations, ceilings, etc, will almost certainly be required. 

What types of extensions are there?

There are many different types, shapes, and sizes of extensions. 

There are four main types or shapes of extensions: 

• Rear extensions. 
• Side return and wrap-around extensions. 
• Double-storey extensions.

Let’s break these down briefly:

Rear Extensions

Rear extensions extend from the rear of the house, usually into the patio or garden. The rear of a house is ideal for an extension, as it keeps the front and rear of the house intact.

Extending onto flat gardens or patios is easiest, but it is possible to extend onto sloping gardens too. Rear extensions provide plenty of ground floor space and a platform to extend upwards into a double-storey extension if necessary. Many choose to extend out the kitchen into a kitchen/diner, adding double or bifold doors to the garden. An excellent choice for those wishing to build a more open-plan ground floor. 

• Rear extensions make use of patio or garden space.
• Probably the most straightforward extension with regard to Planning Permission or Permitted Development.
• Can extend into a double-storey. 
• Great for creating open-plan kitchen-diner living areas. 
• Suitable for most types of homes. 

Side return and wrap-around extensions 

Side return extensions are extremely popular for city terraces that feature a channel to the side of the rear of the house, which often borders a kitchen. Side returns are typical of townhouses, terraces and city semi-detached houses. 

If you have a disused space in the “L” of your property, you can fill that in with a side return extension. Some side returns are large - enough to create a whole new room - but most are reasonably narrow (approx 3 to 4m across). 

Side return extensions are popular for extending kitchens into kitchen-diners, extending living rooms, or for creating open-plan living areas. 

Wrap-around extensions take things a step further by extending at the side and rear of a property in an L-shape. 

• Makes use of the passage, alleyway or side return at the side of the rear of a house. 
• Great choice for houses in heavily built-up areas.
• Popular for extending kitchens and living rooms.
• Open plan potential.
• Suitable for terraced city homes. 

Double or second-storey extension 

Double-storey extensions either add a new storey onto an existing storey or add two new storeys. Some detached or semi-detached houses have space above the kitchen, living room or other parts of the downstairs that can be extended into. 

Another popular option is extending above the garage. Double-storey extensions are pretty cost-efficient as they build on top of existing foundations, which may not need to be strengthened if the home is sufficiently modern. PDRs have recently expanded to cover some types of double-story extensions. 

• Creates extra space for bedrooms.
• Often built on top of existing rooms.
• Can be constructed atop garages.
• Probably more likely to involve Planning Permission.
• Suitable for semi-detached and detached houses. 

Planning Permission for extensions

Permitted Development Rights (PDRs) provide rights to homeowners in England and Wales, enabling them to make alterations to their homes without Planning Permission. Under PDRs, extending your home without going through the official planning process may be lawful. 

Permitted Development applies only to the ‘original fabric of the building’. In other words, if the home was previously extended to its limit under PDRs, you can’t extend it again without Planning Permission. 

Homes qualify for PDRs if they meet the following criteria: 

• Cannot be a flat, or a building containing one or more flats. 
• No extra storey previously added to the home. 
• If the home was converted from a non-residential building to a house (a change in Use Class), PDRs don’t apply. 
• The home wasn’t built as a ‘New Dwellinghouse’ under PDRs .

Specific limits to PDRs apply to buildings built on Article 2(3) designated land, including: 

• Conservation areas.
• National Parks.
• Areas of Outstanding Natural Beauty (AONBs).
• Norfolk and Suffolk Broads.
• World Heritage Sights.
• Some conservation areas and sites of specific scientific interest (SSSIs).

In these situations, it’s absolutely essential to contact the Local Planning Authority to find out their stance on extensions. There’s a fair bit of local interpretation involved in planning laws, especially outside of built-up or metropolitan areas. 

Remember, if your build is listed, then you’ll need to obtain Listed Building Consent for the work. 

Providing your home meets all the above criteria, standard PDRs for extensions are as follows:

For all extensions:

• The extension can’t be built higher than the original roof or 3m within 2m of a boundary.
• Can’t alter the original roof.
• Can’t extend outwards from the front of the house when facing a public highway. 
• Extensions can cover a maximum of half of the freeholder’s land.

For side extensions: 

• Can’t exceed a height of 4m.
• Can only extend half of the width of the original house. 
• Single-storey only.

For single-storey extensions:

• Can’t extend beyond the rear wall by more than 4m for a detached house or 3m for other houses.
• Can’t exceed a height of 4m.

For more than one storey: 

• Mustn't extend beyond the rear wall by more than 3m.
• The roof pitch must try to match the original roof.

By obtaining Prior Approval via a special neighbour consultation scheme (not Planning Permission), It’s possible to extend further than the above:

• Over 3m and up to 6m for all other houses.
• Over 4m and up to 8m for detached houses.

In this situation, you must contact the Local Planning Authority, who will submit your plans to neighbours who have a chance to object. 

Working with a company that provides expert advice is invaluable when you’re trying to work out if your plans fit into Permitted Development or whether you need to apply for Planning Permission. 

Fast Plans will help determine whether your plans fall under PDRs. If not, we will manage the planning application for you, taking away the stress of navigating the fine print of these pretty convoluted rules and regulations! 

Lawful Development Certificates

You can apply for a Lawful Development Certificate (LDC) from the Local Planning Authority for a small fee. These legally declare that the development can go ahead. 

Many contractors require them before beginning work. LDCs are required when seeking Prior Approval for developments in restricted areas, or when neighbour consultation is required. 

It’s strongly recommended to obtain an LDC, and there’s practically no downside to obtaining one. 

Party Wall Agreements

Party Walls are walls shared between a home and its ‘adjoining owners’ (i.e. neighbours).

Terraced and semi-detached houses are pretty much guaranteed to have at least one Party Wall. 

The Party Wall Act provides a legal pathway to developing at or near these shared walls. Since your neighbour also owns the wall, certain works to it involve their consent. This is also true when building foundations near a Party Wall. 

You can view our ultimate guide to Party Walls here which contains all you need to know. 

For works that involve Party Walls, you’ll need to serve Adjoining Owners with a Party Wall Notice. Neighbours have 14-days to respond, with a non-response considered objection (aka. dissent). If you can’t reach an agreement with the neighbour(s), a Party Wall Surveyor will need to carry out an objective assessment. The surveyor will either decline the plans or approve them and provide a Party Wall Award.

If your work involves Party Walls, plan well ahead and prepare to obtain a Party Wall Award if neighbours object. If you get on with your neighbours and your extension has no real impact on their property, an agreement will likely come through within 14-days. But, if you can’t find your neighbours or they continually object, then be prepared for a longer process that can take 2 to 3 months. 

Building Regulations

Aside from planning law, any extension will need to comply with Building Regulations. Building Regulations for extensions can be straightforward or complex, depending on the extent of the works. 

There are tons of Approved Documents specifying regulations for everything from building security to electric car charging points and sound insulation. Of course, not all regulations will be relevant to your extension. 

Here are key Building Regulations to bear in mind: 

• Key Documents A, C and L: Part A regulates the structure of an extension, including foundations. Part C and L specify damp resistance, insulation and energy conservation. The foundations, walls and ceiling need to meet certain specifications to provide structural stability, repel dampness and retain heat. Sub-floor ventilation, damp proof membranes and sufficiently strong joists are required for the ground floor. A, C and L also make provisions for external walls, including thermal insulation.
• Additionally, parts A and B specify that steel beams are required to support ceilings and walls if a room is being opened up into an open-plan space, which is often the case for ground floor extensions. Finally, A, C and L specify what’s required for roof structures, ensuring weatherproofness, insulation and structural stability. 
• Key Documents B, E, F, K, L and Q: Windows and doors are extremely important for Building Regulations as they provide emergency exits and ventilation. For example, Part K makes provisions for preventing falls from windows and balconies by installing safety rails. Part E 
• Key Documents B, F, G, H,  J, L, P: These regulations cover drainage, ventilation and hygiene, which are essential for kitchens and bathrooms. Part B specifies fire safety, which is particularly important for open plan spaces and second-storey extensions. It’s sometimes necessary to fit sprinklers and extra-fire-resistant panels into walls when building open-plan rooms. 

While Building Regulations seem complex, working with reputable surveyors, architects and contractors ensure Building Regulation compliance. Complex extensions are inspected throughout the construction process, ensuring the building remains compliant. 

How much does a house conversion cost?

The costs of house conversions vary hugely depending on the size and complexity of the extension, the location, site access and the quality of fittings and interior works. Location is one of the greatest cost influences, with extensions in London and the South East costing significantly more than those in the north of England and Wales. 

Here are the main cost factors involved in a house extension: 

• The type of extension. 
• The age of the property. 
• Location in the UK.
• Foundational works required. 
• Building Regulations costs. 
• Plumbing.
• Electricity. 
• Fitting an en-suite, toilet or bathroom. 
• The specifications of the insulation and windows. 
• Planning Permission and Party Wall Agreements, where applicable.
• Demolishing and/or relocating walls. 
• Interior design and fittings. 
• Any staircases required. 

The average costs of the three main different extensions are below. As a rule of thumb, expect to pay around £1,500 to £2,000 per m2 of extension. 

• A small extension is around 15m2, likely costing around £22,500 to £30,000. 
• Medium-sized extensions are around 25m2 (which is considered the average extension size). This will likely cost around £37,500 to £50,000.
• Larger extensions of around 35m2 will likely cost around £52,500 to £70,000.

In reality, prices vary with the type of extension, location and quality of design and fittings. Here is a table detailing the average costs of house extensions for standard and high-end extensions.