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A homeowner’s checklist for building an extension

One of the best ways to add value to your home is by adding space.

And with the pandemic seeing homeowners rethinking how they use their homes, it’s no surprise to see extensions becoming more and more popular.

But undertaking major work like this requires a great deal of thought and many homeowners simply don't know where to start.

In this guide, we’ll explore everything you need to know about building an extension…

Building an extension: Where to start

The first thing to establish when building an extension is what your needs are and your reasons for wanting to extend.

If you’re thinking of extending because you need more space, consider where you are most in need of additional square footage and how you use your property’s existing space.

And if you’re contemplating an extension because you want to add value before selling your property, you’ll need to put yourself in your buyer’s shoes.

Ask yourself who your potential buyers are and think about the kind of space would appeal to them.

Will an extension add value?

Ask yourself right from the start whether your plans for an extension will add value to your home if and when you come to sell it.

Sometimes properties in a certain location have a ceiling price, so it can be worth speaking to your local estate agent to get an idea how much value major work might add to your home.

Do I need permission to build an extension?

Most extensions don’t require planning permission, as they often fall under permitted development rights.

However, it’s important to know what you can and can't do under permitted development before you start planning your dream extension project.

Under permitted development, you can do the following without requiring planning permission:

  • Extend from the rear of your detached property by eight metres for a single-storey extension or three metres for a double-storey extension
  • If your property is terraced or semi-detached, you can extend to six metres to the rear of the property for a single-storey extension
  • Build a single-storey extension no higher than four metres to the eaves and the overall height no higher than the existing property
  • Build a two-storey extension if it is no closer than seven metres to the rear boundary

Any extension must not take up more than half of the original land around the property without planning permission and extensions should be built with similar materials to the original property.

There may be different rules for properties in conservation areas or if a building is listed.

So, always speak to your local authority planning department before starting work, to establish if planning permission is required for what you want to do.

Building regulations for extensions

Any property extension must comply with building regulations.

That means whoever builds your extension must be accredited by an official governing body, such as FENSA for window fitters and Gas Safe for gas engineers, or be able to liaise with the local council who will certify the work.

Failing to comply with building regulations means you could be forced to remove your extension at great cost and inconvenience, as well as being unable to sell your home in the meantime.

Buildings insurance and extensions

Before starting work on an extension, contact your buildings insurance provider to let them know about your project.

As the extension will almost certainly increase the rebuilding cost of your property should it be destroyed by fire or flood, your buildings insurance policy will need to be updated.

Your existing policy may cover you, but it's always worth checking and being prepared to pay an increased premium for any additional cover.

Neighbour disputes

One of the most common causes of friction between neighbours can be building work.

Should you need planning permission for your extension, the local authority will contact your neighbours for their views.

However, it’s often a good step to let your neighbours know about your plans before the planning department.

Outline exactly what you're doing and try to provide peace of mind if they have any concerns.

Not only will this keep relations positive, it could also mean they are less likely to oppose the plans with the local authority.

The Party Wall Act

If you’re planning an extension and your property is connected to another property, or properties, you may require a Party Wall agreement with your neighbour(s).

Party walls are shared walls that divide the homes of separate owners, but also include garden walls built over boundaries and excavation work conducted within a certain distance of a neighbouring property.

A Party Wall agreement may be needed if your extension requires groundwork or the use of steel supports that could affect the structure of a neighbouring property, or properties.

How to build an extension – your checklist

Here’s everything you’ll need to do to get your extension started…

1. Speak to an architect

There’s no legal requirement to use an architect to design your extension.

But doing so can often mean a better result.

Architects are trained to make better use of space when designing your extension, so if you have specific needs and goals you want to achieve, it could pay to have a professional in your corner.

However, you'll need to add their fees into your budget.

2. Picking a tradesperson to do the work

This is arguably the most important decision you'll make around your extension project.

Start by speaking to friends and family for recommendations.

Even if they haven't used a particular builder themselves, they may be able to put you in touch with someone they know who has.

There's no better confirmation of a tradesperson's work than word of mouth, even in this digital age where reviews and comments are readily available online.

But do use the Internet, too.

Websites such as Trustpilot or Checkatrade, which both vet their reviews for accuracy and honesty, are great places to start.

Shop around for a builder, of course, but picking the cheapest doesn't always pay off in the long run.

Get quotes from at least five companies and match them up with your budget for the work.

If you're particularly drawn to a more expensive quote based on a builder's previous work or great reviews, only stretch your budget if you can genuinely afford to do so.

3. Finalise your budget early

Staying on budget and not overspending on your extension is crucial if you’re looking to add value to your home.

Make early decisions on what you want and stick to them.

Changes to design or to fixtures and fittings once work is under way will only cause your costs to rise.

And remember: Having a small fighting fund set aside can really help, because major building work can sometimes spring some surprises.

4. The build

Although almost all extensions are different in some way, most will follow a similar process through the construction process.

Here's what you can expect:

  • Site security, storage, access, and risk assessment
  • Demolition (if any), groundworks and drainage
  • Erection of walls and roof
  • Window / door fitting
  • Fitting of downpipes and drainage connections
  • External finishing (rendering, painting)
  • Internal renovation work
  • Electrical / gas / plumbing fit-out
  • Flooring and internal decoration
  • Final clean and handover

The cost of building an extension

Factors that can influence how much your extension costs include:

  • Design specification
  • Planning costs
  • Size, shape and height of the extension
  • Groundworks
  • Issues with trees
  • Site constraints and access
  • Fixtures and fittings
  • Windows and doors
  • Number of trades required

As a rule, you should factor in between £1,200 and £2,500 per square metre for your extension, although very high specification finishes can be as much as £3,000 per square metre.

What is the cheapest way to build an extension?

Building an extension will always cost a considerable amount of money.

However, there are things you can do to help minimise the costs, including:

  • Opting for a simple configuration rather than a complex design
  • Using simple building materials
  • Choosing less expensive interior fittings
  • Positioning plumbing and waste close to existing systems
  • Keep groundworks simple and site extensions away from trees, drains and sewer systems

You can also potentially lower your planning costs by:

  • Designing an extension that falls under permitted development
  • Ensuring a larger extension is acceptable to planners before you apply for formal permission

How long does it take to build an extension?

A small single storey extension of around three metres should take between three and four months to build.

A larger extension, or one over more than one storey, could take around six months.

However, the time it takes to design an extension and obtain any required planning permission often means the overall process takes much longer – sometimes up to 12 months.

Further reading…

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