The Cost Of Prefab Extensions

All you need to know about prefabricated extensions including costs of installation, materials, labour and time frames.

The Cost Of Prefab Extensions

What the job entails

The cost of a house extension, whether traditional and prefabricated, can vary considerably depending on a huge range of factors, but it can often be cheaper than moving. The overall size and style obviously have a large impact on the total cost. prefab or modular extensions are seen as an ideal, cost effective way to add additional space to your home. Prefabricated building extensions are practical and quickly installed, plus they can be made to blend in seamlessly with your existing home.

It's important to remember that an extension costs much more than just the cost of the structure itself. The total costs will include planning, digging, excavation/foundations, down pipes, roofing and tiling, decorating, plastering, flooring, heating, skip hire, doors and windows, and possibly new support beams. There may also be additional costs related to financing and insurance, plus any architects and project managers will need to be paid. Underestimating the total costs is a common mistake (as anyone who has ever watched an episode of Grand Designs will know), so don't forget to include a budget for internal decoration, removal of waste and between £5,000 and £10,000 for planning and preparation costs.

Prefab building extensions are usually cheaper than a traditional extension, but will still add to the house value. A prefabricated modular building extension once finished will blend in seamlessly with your existing home and once finished no one would even know that its prefabricated. Modern prefab extensions are nothing like those ugly, grey concrete slab buildings you may be thinking of. There are many benefits to prefabricated extension building in addition to cheaper costs. Prefabricated extensions allow far greater control of your home as they are built in sections offsite, so you avoid much of the hassle of extension building as the finished module is delivered to the site already made and just needs to be added to the existing building and the new foundations.

Pre-fabricating buildings reduce the total cost of labour as it doesn't take as many man hours for the installation. Many prefab extensions can be fitted in just a few days and they are usually constructed of environmentally materials meaning a lower impact on the environment. They also tend to be better insulated and soundproofed too which means lower energy bills and better comfort.

Because the prefab structure is built in a factory you can typically expect just 6 to 8 days of building work onsite, as opposed to a traditional extension where your home is a messy building site for months. Prefabricated extensions offer a more private living space compared to conservatories and they are also a lot more durable and will outlast conservatories and timber clad type extensions by decades. A prefab or flat-pack extension will cost between 10 and 25 percent less than the traditional extension once you factor in the foundation and excavation costs, plus you get much less noise and impact on your everyday life.

The Cost Of Prefab Extensions

Ordinarily, the cost of extension are often between £1,200 and £1,700 per square metre. So, often can be around £30,000 for the total project of around 20 metres squared. This would be before additional tasks; painting, flooring, lighting, etc. This may vary depending on the sourcing of materials and, potentially, the regional price differences.

At a typical tradesperson's rate would be £200-£250 per day. So for a prefrabricated project being around a week of work can cost between £1400-£2100 accordingly, but with the possibility of additional tradespeople required for such alarge project, this can also incur an additional cost proportionally. It is best to discuss the approximate costs of tradespeople involved and the expected time of completion

Below are some estimated costs of prefabricated extensions.

Job Description Avg. Cost Duration
Prefab extension 9m x 3.5m to create 3 extra rooms supply only £30,000 6-8 Days
Conventional extension 9m x 3.5m to create 3 extra rooms total cost £80,000 2 Weeks

Cost Breakdown

Individual costs of a typical prefabricated extension (around 20 metres squared) - Total Cost: £30,000






Waste Removal


This depends on what type of extension you have ordered. For example, you can include a fully fitted bedroom, bathroom or kitchen complete with appliances if you like. The extension is then made in the factory and everything added ready to be built onto the home in the desired position. A completely finished building which basically just needs to be connected to your existing home then it is usually clad to match the external appearance!
A typical prefab extension is constructed off-site and then delivered to your property and put together in 4-5 working days. But you need to get the foundations completed before delivery so that a crane can simply place the extension on the foundations. Plus, most homeowners prefer to have the exterior of the extension clad or rendered to match the rest of the property which takes another few days.
Flat pack extensions are slightly different as they come in several pieces which have to be put together on-site, so they take slightly longer on site but are often even cheaper.
Much less noise and mess on site as most of the work is done in a factory. Cheaper to buy, cheaper to install, more flexible and generally better insulation. The stress of having a conventional extension built on your home should not be underestimated. The inconvenience of a few days work on the foundation and the prefab extension is nothing in comparison.
Yes - there can be issues as not everyone is a fan of prefab construction, so it can sometimes be tricky when planning permission is required. Prefab constructions are also designed with a shorter life-span, typically around 40 years. Lastly, there is no potential to further extend if you realise you need more room after the original extension, unless you use certain modular prefab extensions from select manufacturers which are designed to be added to.
Structural Insulated Panels (SIPS) can be considered a further variation of timber frame construction as they use timber-based boards with a rigid insulation between. They are very strong panels which are well insulated and are generally used plasterboard in a similar manner to conventional timber frame buildings. So, they may offer a quicker construction, but they are not the same as a prefab building which is completed offsite.