Building a House Extension: A Step by Step Guide

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Building an extension on your house is an excellent way to improve your home. You’ll gain light, space and comfort.

When moving house means hassle and costs, from stamp duty to legal fees, extending your house sounds like an excellent alternative.

This guide will help you:

What to consider before you start

Before you start building an extension, it is important to collect all the preliminary information you can find. You will want to understand all the options you have, from a technical, planning and financial point of view, and then pick the option that is best for you and what you are trying to achieve.

The main factors for a house extension are:

  • Type of extension: single or two-storey? rear, side or wrap-around extension? Would you consider using your loft and/or basement to extend your house?
  • Regulations: can you build under permitted development or do you need planning permission?
  • Party wall: do you need to reach an agreement with your neighbours under the Party Wall Act?
  • Access and parking: will you have easy access to the building site workers and materials? Do you have space for deliveries and vehicles? Will you need to use council space for scaffolding or other temporary structures?
  • Design: what is the best use of your available space? What type and size of windows, roof lights, doors etc? What do building regulations require?
  • Costs and value: What is the best investment to increase the value of the property? How can you add value and have a better living space for yourself and your family?

You might consider hiring an expert in-house extensions to help you understand all the options so you end up making the best decision.

Different types of house extension

Rear extension

It’s an extension at the back of a house, often in the garden. It’s a good option for detached and semi-detached houses. Terraced house extensions at the rear of a property are sometimes feasible, too.

Side return extension

It uses the narrow space at the side of a house, without using up garden space. It’s a great option for a ground floor extension, to implement a change of character on a ground floor without compromising the back garden area. This is an option for semi-detached or detached house extensions.

Wrap around extension

It’s the combination of rear house extension and side extension. It’s feasible for semi-detached or detached house extensions.

Single-storey extension

It’s an excellent way to extend a terraced house, a semi-detached or a detached house. With a single-storey extension, you can also add a sunroom or a conservatory, or add a playroom or a new TV and sofa area to an existing living room.

Double storey extension

It extends the ground and the first floor of a house. It allows you to extend a ground room floor, most usually the kitchen, and to create additional space on the floor above, for example enlarging or creating a bedroom or a bathroom. Most commonly it is feasible as an extension of a semi-detached or detached house.

Loft and basement conversions

Basement and loft conversions are also types of house extensions with specific requirements and regulations. An expert can advise on the options for this type of extension.

Do I need planning permission for a house extension?

There are some extensions that are allowed under permitted development rights. In those cases, you won’t need to apply for planning permission.

In any case, you will need to notify your council about your plans, so that they can update their property records.

For home extensions, there are some basic constraints for permitted development:

  1. Area: the area of side and rear extensions must be less than 50% of the total free area around the house
  2. Appearance: the exterior appearance must blend in with the existing house and the neighbourhood
  3. Width: the width of side extensions must be less than 50% of the width of the existing house
  4. Length: the max length of rear extensions is 3m for terraced or semi-detached houses, up to 4m for detached houses
  5. Height: the height of single-storey extensions must be less than 4m. For two-storey extensions, the height must not exceed the height of the existing house eaves. If the distance to the boundary is less than 2m, only single-storey extensions are allowed under permitted development.
  6. Balconies and terraces: they are never allowed within permitted development rights
  7. Loft and basement conversions: they have specific permitted development requirements. You might consider seeking an expert’s advice if you are planning a loft or basement conversion.

If your property is inside a conservation area or any other type of designated area, or it’s located in a listed building, rules for permitted development are stricter.

Some permitted development work is still allowed, though, You might seek an expert’s advice for planning regulations for a listed building or inside a conservation area.

Building regulations

Even when planning permission is not required, your extension will have to be compliant with building regulations. You will need to create building regulation drawings with all the details to be submitted to the Building Regulation Officers.

You will need a designer or an architect to produce the drawings for you. They will make sure the plans are compliant with building regulations.

Requirements include aspects like structural integrity, fire safety, energy efficiency, damp proofing, ventilation and other key aspects that ensure a building is safe.

After the completion, you will have the Building Regulation authorities to produce a certificate to state that the works have been carried out in line with all the requirements.

Understanding the Party Wall Act

The Party Wall Act regulates alterations of walls or other structures that are sitting astride the boundary between two properties.

Specifically, in the case of house extensions, a section of the Party Wall Act applies, that regulates excavating up to 6m from the boundary.

If your house extension falls within those limits, you will need to give notice of the work to your neighbours. In case they don’t agree with the plans or they fail to reply, a dispute arises and you will need to appoint a Party Wall Surveyor to draw up a Party Wall Award. The surveyors from all parties involved have the duty to resolve matters in a fair and practical way.

On the website you can find an explanatory booklet with all the practical details you need to know about party wall agreements and disputes.

What insurance do I need if I am extending my home?

It’s important to inform your insurer that you are planning to have extension works at your property. Usually building insurance doesn’t cover for events occurring during major building works by default.

Another thing to consider is that most likely the value of your home will increase after the works. For that reason, the insurance policy might need to be updated, including max coverage, excess and premiums.

Of course, you will need to make sure that your building contractor has professional indemnity insurance, public liability insurance and employers liability insurance.

In addition, you might consider further protection for your property during the works, for example, unoccupied building insurance.

Design in efficiency early on

House efficiency is a great benefit when planning building works in your property.

Beyond the Building Regulation requirements, building an efficient extension will lower the running costs of your property.

It will also make it more valuable on the property market, should you decide to remortgage or sell in the future.

Updating the access

Access is a crucial matter when building an extension.

You will need to make sure workers and materials can enter the construction site. Take into account disruption to the neighbouring properties and public areas.

Access can also be an issue in busy areas. House extensions often mean more people and more cars, so you might be required to provide extra parking spaces or to prove that the extension won’t affect the local traffic.

Seek professional advice if you think your home extension has access issues before, during and after construction.

Project management

To save on costs, you can choose to be the project manager of your own house extension. You will need to liaise with the designer, the council, building control before and during the project; then you will have to find and hire builders, supply materials and (maybe) tools and direct the working site.

Especially if there is substantial structural work involved, or your home extension is not a plain standard project, you might want to seek advice or hire a main contractor to carry out the preparation or the building work or both.

Living on-site could slow down progress

Any good contractor will suggest that you move while the work is in progress. Consider that legally the contractor is responsible for the site, and no one but the workers are allowed to enter a building site.

When you plan for your budget for house extensions, make sure you allow for a temporary accommodation. Make sure to hire professional builders who have a clause in the contract to provide for the temporary accommodation costs, if there are delays in the project schedule.

Bigger isn’t always better

When doing major building work in your property, you might be tempted to plan for the biggest size you can add.

Consider that a clever design adds much more value to your house, both in terms of quality of life and in terms of market value, should you decide to sell, rent out or remortgage.

This is even more important when you want to stay within the permitted development rights.

Make sure you hire the right designer for your extension: it can make a huge difference in the final results.

Building an extension

Once the design, plans, party wall and permissions are sorted out, you will need to find the right company to carry out the work.

Whether you decided to hire a main contractor or to be the project manager and hire builders and tradespeople yourself, search thoroughly and read all the info you can find on your candidate workers.

Direct referrals from a friend or a neighbour are good starting points.

Remember that any contractor or builder is able to give you a detailed breakdown of costs and to quote a fixed price for your extension. Avoid agreeing on daily rates, as this will make the project slower and the cost higher.

Heating and electrics

Building an extension can be a good opportunity to upgrade your heating and electrics, if not always necessary. There are cases where, adding an extension, your boiler or your heating system won’t be powerful enough to supply the additional space, pipes and wires.

You can use the opportunity to replace your old sockets and have double ones or to install outdoor lighting.

How much does it cost to build a house extension?

To be able to budget for house extensions, it’s important to have an idea of the average costs. Consider they can vary according to the property market price per square metre.

The average cost of a standard house extension can be £1500 to £2500 psm as a UK average, including finishes. The cost can go up to between £2,000 and £3,000 per sqm in London, excluding finishes like windows, flooring, sanitaryware etc.

In general, the cost per square meter of an extension goes up and down with the cost per square meter on the property market for the area.

When planning your budget, remember you will have other expenses before the building cost, for example, design, planning (if applicable), structural calcs, building regulations, insurance, cost of moving out etc.

If you are looking for cost estimation, houseUP is an extension specialist company that gives free site visits and free cost estimation for house extensions.

How to build a budget extension

If you want to extend on a budget, there are some great ways to save money:

  • Choose the simplest design, so that you can hire a consultant for plans and drawings
  • Handle the party wall surveyor costs by liaising with the neighbours directly and well in advance
  • Choose off-the-shelf products
  • Hire tradespeople and supply materials directly
  • Choose a builder who gives you a fixed price with a detailed breakdown and avoid extras along the way
  • Minimise the changes to services, especially water, sewage and manholes

Remember that sometimes it’s not worth saving on who you hire, as experienced and competent professionals can prevent issues and save you money overall.