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How to go about building an extension

Building an extension can be a sure fire way to add space and value to your property. From planning permission to costs, here’s what you need to know.

With the hassle and expense of moving house – from stamp duty to solicitors’ fees – the reasons to improve your existing property soon mount up. An extension is one of the most obvious ways to add some much-needed space to your home, and in the majority of cases, it’ll also add value. But as far as home improvements go, it’s a bit of an undertaking. From costs and design to insurance and building regulations, there’s a lot to consider to make sure your extension meets your needs and obtains planning permission. Here we cover everything you need to know when building an extension.


The basics

Building a single storey extension is a cost-effective alternative to moving house, allowing you to design additional living space with a layout that suits how you use your home. With expanses of glazing, it can also provide you with the opportunity to create a light-filled space that’s integrally linked with the garden.


Is it the right move?

Moving house isn’t feasible for everyone when there are agent fees, legal fees, the deposit, and stamp duty to think about. The move itself can be a hassle, plus you’ll likely pay a premium for the desired extra bedroom or bigger kitchen. You might find that you can build a decent extension, adding both space and value to your existing home, for a similar price or less, and without the upheaval. As with any big home improvement project, it’s worth crunching a few numbers before you dive in. An estate agent will be able to guide you on whether your plan will add value to your home. They should also be able to give you an indication of how much it would increase the value by. If you can’t book in time with your agent, figures from the Office for National Statistics have been put towards this calculator that will help with financial estimates. In most cases, if you are able to extend, it is often sensible to do so from a financial point of view. The cost of buying a new house will typically outweigh the cost of building. However, the final result needs to be balanced, rational home. For example, a five-bedroom family house with a small living space, or a house with a huge kitchen but tiny bedrooms, could be problematic if you need to sell in the future. Even if you have no intention of moving for the foreseeable future, it’s best to avoid spending a lot of money on expensive building work that could make the property harder to sell.


Things to consider

Shared walls

Under the Party Wall Act, you’ll be required to tell your neighbour if you plan to build on or at the boundary of your two properties, work on an existing party wall or structure or dig below or near to the foundation level of their property. A party wall is the shared wall that divides the homes of two separate owners. It also includes garden walls built over a boundary and excavations close to a neighbour’s property (typically within three or six meters). You must give your neighbour at least two months notice before you start work and you’ll likely need to appoint a surveyor to arrange a party wall agreement.

Site access

Trucks and lorries will need access to your property to unload large items and materials. You’ll need to consider site access, parking and potentially storage supplies and tools.


It’s important to notify your home and contents insurance provider of the work before you start. Some may not provide cover during the building process, but others offer dedicated extension insurance plans. Building an extension will likely increase the rebuild cost of your house – which insurers take into account when pricing premiums – while any building work could put the property at risk of damage. Your insurer will let you know if your current policy will cover the new extension; you may find your premiums increase. If you don’t let your insurer know about your extension and there’s a problem with the property at some point, you may find your policy is void.

Demands on your boiler

Is your existing boiler going to be able to cope with more rooms? Replacing the boiler may be a necessity, unless you look at alternatives such as underfloor heating.


Planning permission

Under Permitted Development Rights, some extension projects can be undertaken without the need for planning permission. Since 2015, fairly relaxed rights have allowed homeowners to have up to 75% more extension space without full planning permission, however on 30 May 2019 this act will again be reviewed and possibly revived. Currently, you will need planning permission if your extension extends towards a road, increases the overall height of the building or is taller than four metres, covers more than half the area of land surrounding your home, extends more than six metres from the rear of an attached house or eight metres from the rear of a detached house, is more than half the width of your house, includes a balcony or raised veranda or uses different materials to those of the original house. Your builder or architect should be familiar with these limitations, but it’s worth double-checking yourself to avoid making a planning application more than once. It’s also worth noting that there are costs attached to planning applications. You can calculate the planning application fee yourself using this online fee calculator.


Building regulations

An extension of any kind must comply with building regulations. This is different from planning permission; you may need both to apply for both. You will need to submit an application for the work to your local authority’s building control department. Each authority has its own table of charges or you can use a private certified building control firm. Some contractors are building control certified, meaning they can carry out work without the need for involving the building control department. These include FENSA accredited window fitters, Gas Safe registered gas engineers and NICEIC registered electricians. If the requirements aren’t met, you could be served with a notice to take the extension down and will have trouble when it comes to selling your home without the relevant Building Regulations certificates.


How much will it cost?

Obviously the size of your extension will dictate the cost, but there are a number of other variables to consider. Most extensions will be subject to VAT on labour and materials at the standard rate of 20%, especially if you use a contractor to undertake the work. It’s possible to save the 20% VAT on labour if you use local tradesmen who are not VAT registered however you will still have to pay VAT on materials at the standard rate. Soil type, what the extension will be used for, glazing specifications and location are all other factors that will affect the cost. It’s worth noting that it actually gets slightly cheaper per unit area as the size increases and a two-storey extension will not cost much more per square metre because, aside from the extra interior fixtures and finishes, you are only adding walls and floor joists — a roof and foundations are required whether your extension is single or two storey.

Are you looking to buy, sell, rent or let? Get in touch.  

Are you looking to buy, sell, rent or let?

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