How Much Does an Extension Cost? | UK Guide | 2023

how much does an extension cost

Adding an extension to your home can be a great way of maximising your living space to accommodate a growing family. It can also be a cost-effective way of adding value to your home. However, building an extension can be a costly project. In some cases, it can be cheaper to sell up and purchase a property better suited to your needs than shelling out for an extension. Need some help deciding whether or not an extension project is the right thing for you? Our guide explores the options available to you and what cost implications are involved when building an extension.

Things to Consider When Considering an Extension

The overall cost of an extension is determined my multiple factors. For starters, you need to decide the overall size of the extension you are looking to add to your property. A two-storey extension is obviously going to cost you a lot more than adding a simple space onto a ground-floor room. The overall size of your extension will also have a huge impact on price, with more materials being required and more man hours needed to construct it. If your property requires an extension to be of an unusual shape, this can also see your building budget soar.

You will also need to think about the overall build quality you are expecting from an extension project. If you are happy with basic materials and minimal flourishes, you can keep your overall budget low. However, if you want a premium finish with all the trimmings, expect to pay a considerably higher amount for the project to be realised.

Another key factor that affects the cost of an extension is the purpose of the space. If you are planning on building an extension to serve as a bathroom or kitchen, you will be looking at a higher overall cost than if the extension was purely to be used as additional living space. Another thing to think about is the amount of windows you want your extension to have. Additional glazing not only makes the general build of an extension more complicated, it also brings the added cost of windows and frames.

If you wan to be involved with the build itself, you can potentially bring the overall cost of an extension down. For example, if you have some relevant experience and can turn your hand to managing a project, you will not have to pay someone to oversee the construction of your extension. However, taking control of a project is not always a good idea. If your inexperienced, you may in fact cause the project to overrun, with unnecessary costs incurred because of a longer schedule. It is often better to leave planning, building and overseeing to the experts if you want an extension project to come in on time and on schedule.

The Cost of a Single-Storey Extension Project

If you r extension plans are fairly basic, you do not need to spend that much money at all. In fact, a humble ground-storey extension with a simple floor plan can cost as little as £1000 per square metre. If you want a more premium finish, you can expect to pay double this amount. If you want a ground-storey extension to serve as a bathroom or a kitchen, you will also need to factor in the added cost of gas and plumbing.

Two-Storey Extensions

Although you would think that two-storey extensions cost significantly more than a single-storey one, the additional premium for such building work is not that much. This is because much of the groundwork is taken care of. With foundations laid for the ground-storey extension, you are only really paying extra for additional floor joists and walls. Of course, you will also need to be pay extra for interior finishes and fixtures, but the price of these is nominal when compared to other costs. Again, if you want the upper levels of your second-storey extension to house a bathroom, you need to factor in the additional cost of plumbing.

Additional Costs

Other than manual labour and materials, your budget should also take into account a further raft of costs. Surveys and planning applications are things you can not avoid when building an extension, so you will have to add the cost of these into your overall budget. You may also need to pay for a development certificate, although the cost of these is fairly small.

If your property is attached to another property, such as a terraced or semi-detached building, you may also need to get a party wall agreement finalised with your neighbours. This can be time-consuming if you have uncooperative neighbours and will also add further cost to your extension project budget. Finally, you will need to ensure any extension project is fully insured.

Management Costs

When it comes to the building of the extension itself, you can determine costings by adopting different build routes. If you are experienced enough to handle the work yourself, taking the DIY route can yield massive savings. The biggest saving you will make is on labour costs, even if you need to source qualified contractors to undertake certain tasks. You can also make big savings if you purchase materials directly from suppliers yourself and cut out the middle man. If you outsourcing your extension project, you will often find that material costs come with substantial mark-ups attached.

If you do decide to outsource your project to subcontractors, you can think about maintaining some involvement yourself. You can still manage the project, even if all tradespeople are handling the building work. As with a DIY approach, think about purchasing materials directly from supplier to make the biggest possible savings on your build.

You can of course turn overall all the work to contractors. You have different approaches available here. You can pick a single supplier to handle all of the work, giving them the option to choose their own subcontractors for the job. While this is convenient, you face uncertainty when it comes to the ultimate cost of your build project. If you want to keep your eye on costings, make sure you have final say on any hiring and purchasing decisions.

Estimates & Quotes

Understanding the difference between quotes and estimates is crucial in ensuring you never pay more than you plan to on an extension. If you are using contractors, bear in min that estimates are just that. They are a general guide on what you can expect to pay, rather than an iron-clad agreement on what you will end up paying. A quote is a different thing entirely. When given a quotation, you are being given an agreed upon price. When you are provided with a quote, make sure it is given in writing so it can serve as a legally binding agreement.

You should also ascertain whether or not a quote includes the additional cost of VAT. Provided you have this, you will have a very clear idea of how much your extension is going to cost you. However, there will be some instances where additional costs arise. There be unanticipated legal developments, or you may wish to make changes to your building plans. Should you decide to amend your approach down the line, again make sure you have quotes provided to do you in writing so you have a clear idea of what your outgoings will be.


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