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A comprehensive guide to building an extension

Getting the planning and preperation right is vital when buidling an extension. For a full guide to designing, planning, budgeting, project management, and the construction of your extension, read on.

Building an extension can add extra space and value to your home. Whether you need a guest bedroom, bigger living room, or home office, an extension can transform your home.

But, an extension needs careful and precise planning, preparation and execution.

Get started with our step-by-step guide to planning and building an extension.

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Where to start building an extension

Doing some initial planning will help your extension build go smoothly.

Here’s a general checklist of things to do before building an extension:

1. Planning permission

Because of permitted development rights, you won’t usually need planning permission for single-storey or conservatory extensions.

However, whether you can build without planning permission depends on your property type, the build, and location.

Check with your local planning authority if you’re unsure.

2. Building regulations

All homeowners must comply with building regulations.

These regulations outline the required standards for safety, structural integrity, insulation, fire safety, and accessibility.

Hiring trades registered with a Competent Person Scheme means they can sign off on work to meet the regulations.

3. Lawful development

Even under Permitted Development, applying for a Lawful Development Certificate (LDC) can be a good idea.

Getting an LDC confirms that the building and the construction work did not require planning permission.

4. Party wall agreement

Your home extension might involve building work near or on a wall shared with your neighbour’s property.

In this case, you may need a party wall agreement.

5. Neighbours

Always consider your neighbours.

Even if you don’t need to worry about a party wall, it’s a good idea to inform your neighbours of your plans and any future disruption.

6. Leasehold or freehold

If you own the leasehold, not freehold, to your property, always check the lease details.

Speak with your freeholder before planning or designing an extension.

7. Home insurance

An extension will likely increase the rebuild cost of your home should the worst happen.

Your home insurance provider may need to readjust your premiums.

Always update your insurer about major home renovations to avoid a void policy and make sure you’re covered.

8. Project management

Before planning or designing an extension, consider who will manage the project.

You can manage your extension build yourself or hire a professional project manager for a smoother process.

Professionals who can help you manage your extension in part or fully include:

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Checkatrade contractor iFurb checking construction plans

Budgeting for your extension

Your home extension can add real value and kerb appeal to your home.

If you’d like to know how much an extension might add to your home’s value, speak with a local estate agent. This can help you set your budget.

Some helpful tips for staying on budget include:

  • Always decide on your budget in advance
  • Add a 20% contingency fund for unforeseen expenses
  • Get multiple written quotes that include labour, materials and VAT to avoid surprises
  • Try to make design decisions upfront about flooring, tiling, fixtures and fittings, and decoration

Finding the right professionals

Finding reliable, experienced tradespeople can help your project run smoothly.

They can guide you through the process to make sure your extension meets your brief and any legal requirements.

Always look for reputable and highly recommended professionals, including builders, architects, plumbers, electricians, and decorators.

Top tips for finding the right tradespeople include:

  • Get multiple written quotes
  • Ask for recommendations from friends and family
  • Get references or check reviews
  • Look for examples of their past work
  • Only hire an insured tradesperson in case of accidental damage
  • Ask what warranties or satisfaction guarantees your tradespeople offer

Step-by-step guide to building an extension

The steps to building an extension can be simplified into several key stages.

1. External construction

With plans drawn up and approved and your dream team in place, construction can begin.

This phase involves:

  • Preparing the site ,e.g. bringing in scaffolding, skips, toilet facilities, and other tools or equipment
  • Excavating the construction area and preparing the groundwork
  • Installing gullies and soakaways
  • Building the structure of the extension, including the foundations, walls and roof
  • Installing windows and doors
  • Adding downpipes and connecting to the drainage system

Scaffolding for two-storey extension

2. Interior work

Once the main structure is complete, your project team will focus on the interior.

This will involve:

  • Addressing any redundant internal fixtures and fittings
  • Reconfiguring your internal space where necessary
  • Installing insulation

Timber frame house extension

3. Internal fit out

Next, it’s the internal fit-out. This includes:

Plastering an extension

4. Interior decor

You’ve probably had a creative vision in your mind for a while. Now’s the chance to bring it to life.

This stage involves:

Two storey extension interior design

5. Final inspections

There are just two last things to do:

  • Manage any snagging issues with your contractors
  • Arrange for final inspections to ensure that your extension complies with all safety and building regulations

Side return home extension

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Building an extension checklist

To help you get started building an extension, we’ve included a helpful checklist.

To recap, the six key stages of building an extension include:

1. Planning and preparation

  • Check you have the correct legal permissions and planning permissions
  • Draw up your budget and designs
  • Speak to your home insurance provider about extension insurance
  • Book the necessary professionals and create a schedule of works
  • Order all necessary materials (unless your tradesperson has agreed to do this)
  • Check there is access space for vehicles

2. External construction

  • Prepare the site and bring in any scaffolding, skips, tools or equipment
  • Excavate the construction area
  • Prepare the groundwork
  • Build the structure of the extension
  • Install windows and doors

3. Internal renovation

  • Remove or replace redundant internal fixtures and fittings
  • Install insulation

4. Internal fit out

  • Plaster walls and ceilings and lay flooring
  • Add utilities, including plumbing and electrics
  • Install fixtures, fittings, and appliances

5. Internal decor and finishing touches

  • Finish your painting and decoration
  • Add light fixtures, storage, furniture, etc

6. Final inspections

  • Manage any snagging issues
  • Arrange for final inspections

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What do I need to do before building an extension?

A home extension is a valuable but costly investment.

To get the most from it, start by considering your living needs and specific requirements.

Some of the types of house extensions you might consider include:

Building an extension

What size extension can you build without permission?

Your extension build should always stay under the highest part of the existing roof.

Single-storey extensions can be no more than four metres high from the original back wall (three metres for terraces/semi-detached).

Side extensions must be a maximum of four metres high, one story and no more than half the width of your house.

See how big you can build without planning permission for a more in-depth explanation.

What is the average cost of building an extension?

The size, scope, and complexity of the work and the cost of the fixtures and fittings you select will impact the price.

A basic 20m² extension could cost between £40,000 – £56,000. A premium extension may cost between £110,000 – £92,500 (excl. VAT).

Take a look at our house extension cost guide for more information.

Take a look at our other cost guides for help pricing your extension:

Do I need an architect for an extension?

Hiring an architect is not a legal requirement for all extensions.

But their expertise can be invaluable because an architect can:

  • Bring professional expertise and creative vision to your project
  • Create initial designs that take into account your needs, available space, layout, and materials
  • Plan the extension to blend with the existing structure and architectural style
  • Help you factor in surrounding trees, flood risk, site access, and rights of way

Your architect can also design a functional and aesthetically pleasing extension while:

  • Handling all necessary planning permissions
  • Develop a design that maximises your available space and preferences
  • Recommend the best, most cost-effective materials and designs
  • Sort complex planning regulations, making sure your extension receives approval
  • Oversee the construction process, ensuring the project stays on track and within budget
  • Add value to your property by delivering a well-designed and professionally executed extension

Read our guide to architect fees.

Can I draw my own plans for building regs?

Drawing plans for an extension yourself is possible if you have architectural knowledge and experience.

But they must comply with the relevant regulations. Any mistakes or oversights could lead to delays or costly alterations.

Accuracy is vital. Your builder will use the drawings to give you a quote for your extension and as a blueprint for the build.

If you choose to draw your own plans, be sure to:

  • Research and understand the building regulations applicable to your project
  • Use accurate measurements and detailed drawings to convey your intentions clearly
  • Consult with a professional or your local authority if you have any doubts or questions

Building an extension plans

Ready to embark on your exciting extension build?

Wondering how to find trusted, experienced professionals for a smooth extension build? Checkatrade can help.

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