How to Plan a Home Extension

Building an extension is a work of passion; it is usually something that you spend months saving and planning for. You put your heart and soul into it and your trust in the tradesmen that you choose to realise all your effort, therefore it is wise to understand every stage of the process, from planning to completion. It is a substantial financial commitment, so it’s good to arm yourself with knowledge to minimise the opportunity to miss or misunderstand any stage of the home extension project, helping it to run smoothly.

In this guide I cover everything you need to know, from planning to completion, before you start on site.


A house built from £20 notes

The amount you need to budget for your home extension will vary depending on a number of factors. Some of these factors are obvious: size, specification, location, but it also depends on where you are in the UK too. A basic extension can cost you between £1000 to £1500 per m2. If you’re looking for a top quality extension it can be around £2000 per m2.

When you are setting your budget for the extension, you may want to research the property prices in your area and make sure you are going to get a return on your investment. Unless it isn’t a priority for you, you wouldn’t want to invest more than your property is worth. Budgeting for a two storey extension wouldn’t cost much more as most of the cost is in the foundations and roof, which is a requirement of any extension.


Check your insurance before the work starts. Your current building and contents insurance may not cover the property during the extension work. They may be able to offer you a solution or you can purchase extension insurance. If you are project managing the work yourself, you should purchase specialist extension insurance as it will cover the new work, as well as your home, for any loss or damage experienced as part of the project. Which means that if part of your home collapses , you are adequately insured. General building insurance doesn’t cover this.

It is wise to discuss the home improvement project with professional insurers as it often includes liability under the Party Wall act. It will also automatically include public and employers liability so you are covered on all bases.

You take out extension insurance to start from the planning stage until you move into it.​


​Shared Walls

Unless you live in a detached property you will need to consider any shared walls. These are the party walls, party wall structure or boundary that is within 3m of your extension or if you are digging foundations within 6m of a boundary that belongs to your neighbour. If there are any then you will need to make sure that the work complies with the Party Wall Act.

Site Access

How easy will it be for suppliers to make deliveries and to get their product to where it needs to be? Do they need to take it though the home? If so, will it fit? If necessary, can lorries get down your road and can tradesmen park outside or store their tools on site?


Will your heating system, plumbing, electrics and gas be able to service the extra space that you are adding? Do you want to take to opportunity to other sources such as underfloor heating?


  • soil conditions
  • services
  • surrounding trees
  • any history of flooding
  • rights of way

Planning permission & Permitted Development Rights

Architect visiting a new build​​Permitted Development rights

Permitted development rights outline what a homeowner is allowed to do to their property without the need for planning permission. Please note, these are valid at time of writing and should be checked by anyone wishing to carry out a home extension project; they do not apply to anyone in listed buildings or conservation areas.

What you can do under permitted development rights:

  • Detached properties can be extended by 8m to the rear for a single-storey extension, or 3m if it’s two stories
  • Single-storey extension can’t be higher than the existing home or more than 4m of the ridge and the eaves
  • Two-storey extensions must be more than 7m away from the rear boundary
  • Extensions to the side of the property can only be single storey, a maximum height of 4m and a width no more than half of the original building
  • The new extension must be built in the same or similar material to the existing property
  • Extensions must not exceed beyond the building line of the original property
  • Extensions must not occupy more than half of the garden

What to do if your plans exceed your permitted development rights:

If you are planning a home extension which exceeds the parameters set out above then you will need to seek planning permission from your local council. Contact the planning department as early as possible to seek their advice and understand their planning policies. Doing this will increase your understanding and help you to create a planning application that will likely get approved. Currently planning applications cost £206 and applications can be made through your local authority or the Planning Portal.

Lawful Development Certificate

A Lawful Development Certificate is always worth having just in case. You can apply for it from your local authority and it costs around £103. It confirms that the work carried out is lawful and meets with the requirements of permitted development.

Building Regulations

Every extension is required to conform to building regulations whether the extension is being carried out under Permitted Development or not. To do this you need to submit one of two things:

  • A Full Plan Submission
  • A building notice

Full Plan

These are plans of the building work that is going to be carried out. You need to send them to your local authority and they need to be approved prior to any work being carried out. Once they have been approved, the building inspector assigned to your project by the local authority will inspect the progress of the work at different intervals

Building Notice

This is a statement that you submit to the local authority to inform them of the nature of the work being carried out and that it will be compliant to the building regulations. This should be submitted to the local authority building control department at the very latest 48 hours before the commencement of work. As with the full plan, a building inspector will be along at various stages of the home improvement project.

It is important to note that submitting a full plan is more comprehensive and you are less likely to discover that you are not fully compliant mid-way through. This can be costly and delay the home improvement project.​

Warranties and guarantees

As with anything in home improvements, it is always wise to make sure you get a warranty or guarantee for the work that you are having done. This is as true as replacing windows as it is with getting a loft extension. This ensures that you are not caught short by unforeseen circumstances, e.g. poor workmanship.

Planning the work

House plansDesigning an extension

There are three options:

  • Design it yourself. You will need to be confident that you can draw up a plan yourself to submit
  • Contract a building company that will draw one up for you. Many loft extension companies often have this service as part of their package
  • Appoint an architect/designer/structural engineer

If you need to apply for planning permission and you have drawn up the plans yourself, you may need to hire a draftsperson and/or a structural engineer to produce drawings and structural calculations to submit.

No matter who draws up the plans, these will form the base for builders to submit their tender.

Useful Websites for Architects

A requirement of these industry bodies is that full members need to have relevant academic and technical qualifications. NB: whoever you choose, make sure they have sufficient indemnity insurance.

Building Regulations

Whether or not you need to apply for planning permission, the elements of your home extension will be subject to satisfying the requirements of their specific regulations, for example, a window on the ground floor will need to be manufactured to a minimum uValue of 1.6 and may need a trickle vent if there isn’t enough or any other means of ventilation in the room.

The Planning Portal has helpful interactive guides to help you understand what is required and to guide you through your project. Whether or not you are designing the home extension yourself, it is a good idea to get an understanding of the regulations and restrictions on building. This could help you understand any limitations which, in turn, will help you to avoid disappointment and headaches later on when you are discussing your requirements with anyone involved with the planning and build.

Finding professionals

Architect and builder discussing project plans

Finding the right professionals is extremely important as you will be working closely with them for a few months and building a relationship with them. It is especially important to find the right builder as they will no doubt be in your home and have contact with you on a daily basis. Therefore you will need to find someone that you like and trust. Ensure that you do your research:

  • Ask friends and family for recommendations
  • Check out websites like to get recommendations from people in your area
  • Check out specialist directories like Houzz or Check a Trade
  • Ask to see their portfolio of work
  • Check out their reviews
  • Ask if you can speak to past clients
  • Check that qualifications are up to date

You want to find professionals who understand what you want to achieve and help you to realistically achieve it. Who explain anything that may be out of reach and ways that it can be overcome. It is important to be clear and often helps if you have drawn out the ideas that you have in your head, along with explanations.

With sufficient information an experienced contractor should be able to give you a detailed fixed price quotation. Try to avoid day rates if you can as they have the potential to be disastrous.

Make sure that any professional you hire has the appropriate insurances, e.g. building contractors have risk insurance.

Extending utilities

Central Heating

One of the questions you should ask yourself when planning a project is: can my central heating system be extended? A central heating system is installed based on the size of the property it is being installed into. In most cases it can be extended, however you will need to understand by how much and if it can handle the size of the extension you are planning. If you can’t extend the central system you have a few choices:

  • Replacing the boiler
  • Add a second system
  • Add an alternative heat source, e.g. underfloor heating


If your extension is going to create a substantial increase on the demand of the electric board you may have to add a circuit; an example of this would be a kitchen. If you are adding a dining room or bedroom then most of the time it is possible to extend the electrics. If you do need to extend, why not take this opportunity to turn single sockets in to doubles and illuminate the garden?

Managing the project

Project planning - checklist

Somebody needs to be in charge and manage the progress of the project. As with designing an extension you have various options:

  • Manage it yourself – hire and coordinate the subcontractors to fulfil the project
  • Employ a specialist project manager to manage it for you
  • Contract a building company that will appoint a person as part of their package
  • Contract a design and build company who will manage the home extension project from plan to completion

There are pros and cons to each option and it depends how much available time you have, how close you want to be to the project and what type of control you want to have and are comfortable with.

Self-managing the project

If you decide to manage the home extension project yourself you will need to manage:

  • Ensuring that the relevant insurances are in place
  • The different tradesmen
  • Material ordering and delivery
  • The services (electricity etc)
  • Keeping the site tidy
  • Disposing of any waste

Managing the different tradesmen is critical to maintain the flow of the home improvement project and keep it moving. The number one factor of budget creep on a project is wasting time as this can increase the cost of plant hire, scaffolding etc. If you are managing the home improvement project then you are responsible.

Should You Move Out?

For a home extension project this is largely down to you. You will need to decide from the beginning whether or not you will be able to cope with the building work and the environment that is creates (dust, mud etc). If you work from home you may not be able to cope with the noise. Another factor is whether or not you would be able to afford somewhere else to live and whether or not you want to go through the disruption that this causes.

If you decide to move out then this will need to be factored in to the budget.


The biggest question when planning any home improvement project is: where is the money is going to come from? If you are extremely lucky you can pay for it outright. You may have been saving for a while but need a bit extra to top it up or you might need to borrow the whole amount. If latter two apply, then there are a few options;

  • ​Credit card for a bit of a top up. If you a good at managing money, you can get good 0% interest deals
  • Personal loans
  • Specialist home improvement loans
  • Remortgage or secured loan


Make sure you understand the payment schedule from the start. General practice is to make a payment to your builder at each stage of the project. Don’t make any upfront payments if you can avoid it.

If larger items are requested to be brought in advance (e.g. kitchen units) ensure that your name is on the order just in case something untoward should happen.

If a designer or architect is involved, they tend to charge a fee based on the project amount. Make sure you understand this fee before you commission the plans. Check if any structural calculations are required and if so, whether the provision for them has been made.


Unless you are building a new home you do have to pay VAT (L). However if you can prove that the property you are extending has been vacant for 2 years, it will be classed as a conversion where the VAT rate is 5%. To get the reduced rate you will need to appoint VAT registered companies.


We hope this has been helpful. Happy building!

P.S. We are not responsible for any decisions the reader makes based on the information in this webpage.