When we give our clients a figure of how much we would expect the extension to cost, it is very rarely what they were expecting. There is a lot of information online about how much to pay for an extension but there are so many differing opinions and data, it is almost impossible to judge. Every builder is slightly different, so their prices will be different too. Also, what you are building and what type of extension you want will have a huge impact on the costings.

One of the worst things for us is bursting a client’s bubble and informing them that they are either going to have to raise extra funds for their build or look at reducing what they are building. But we would rather go through this process and get planning permissions for clients on designs that they can afford to build, as there is nothing more frustrating than building your hopes up and falling in love with a design to find out it is drastically out of budget. However, just because a project is over budget it does not always have to result in drastic changes – there are several things you can do to reduce the build cost of an extension.

Builders will always look at the size of an extension when working out the cost of the project. If you are building an extension at 4m in depth and are way over budget, the obvious thing to do is to reduce the depth of the extension. This is going to have an impact on the price, but it does not always make a drastic difference. It isn’t always the size of the extension that is driving the price up. Often it is the complexity of the build, or the amount of steel work that is needed – maybe the items in the kitchen are too much, or the materials that have been selected for the finish of walls. A pitched roof on an extension can be up to £5,000 more than a flat roof; bespoke size doors and windows can add thousands to the build. If you are looking at opening one room into another and removing a wall, the steel work and labor can add to the build price, whereas you could look at creating an opening in the wall to allow access between each room; rather than steel, a concrete lintel could be used, or rather than creating a new opening, an existing doorway can be taken out to open one space into another.

A project that over-runs can have a drastic effect on the costs of the build – it isn’t always the cost of materials that puts the price up. Some of our clients move out during a build or haven’t moved into the new house yet and are paying mortgages or rent. Every month the project over runs is another month of payments that must be made. We see a lot of people project managing their own builds and in most cases it is ok; if you do misjudge an order or cause delays by booking in a tradesmen at the wrong time, you can end up paying someone to work who cannot get to site and do what you want them to.

It is so important to get planning permission for a design that can be built, and for one that you want to build. It is much easier and cheaper to make changes to a drawing than it is to make changes on site or re-do work because something has not been considered during the design stage. We are aware that most of our clients have never carried out an extension project before, and even more have never actually built anything themselves. Our aim is to help our clients get a space that they love and that works exactly how they want it to; it is easy to go along with what the client wants, but it is our responsibility to ensure that they love the space that is designed, but also that it can actually be built. It is easy on paper to remove walls and change structural elements, but it is not always practical and can easily add vast amounts to the build price. When selecting an architect, designer or draftsmen it is important that they have carried out designs for similar extensions. Spend time with your architect or designer going over the space; make sure it works for you, but make sure it can be built. It’s much cheaper to spend a few extra hours on the design than to have tradesmen standing around on site.

It isn’t always the size of the extension that is making the build expensive, it can also be the location of it or the location of trees and sewer pipes. It can be very expensive to move drains, especially if they run onto neighbours land. If you are building near or over a sewer, you will need to have permission from Thames Water. If one of the walls is running directly over a sewer pipe, it can cause a real problem. On some properties you can not move the sewer pipe, which means you need to go back to the council and get approval for a bigger or smaller extension to avoid the sewer pipe. If this cannot be done, you must build a new structure under the ground out of steel to avoid having to have a foundation. Not only will this increase the bill from the structural engineers, it is a complicated element for the builder to construct, which will increase the length of the build but also increase the cost. Trees that are close to extension can also have an impact on the design and the cost. If tree roots are near a proposed foundation, you may have to use steel beams to avoid these, or used piled foundations which, again, will increase the cost of the project.

There are a lot of elements on a build that we could all do ourselves – we could all spend a weekend breaking up a patio or removing a shed that is in the way. Some of the internal finishes, like painting and tiling do not always have to be done by a builder. I would recommend that a skilled tradesman completes the whole build, but if you are running over budget and do not want to make drastic changes to the design, you can save money on the build by carrying out some of the work yourself. You can also source some of the labor or may have friends that are tradesmen and can help with some elements of the build. You may have doors or windows that you have not long purchased, these could be re used in your new extension, likewise a kitchen does not always need to be replaced, even if you are proposing for it to be in a new location, the units you have can be moved, rather than a whole new kitchen you could just look at purchasing a new worktop or new handles to give the kitchen you have a new lease of life. If you are planning to have additional bathrooms or toilets, you can get the builder to run all the pipework and electrics, so everything is in place; at a later date, you can purchase the sanitaryware and tiles, and this can take thousands of the cost.

Before you select your builder, get a full breakdown of the project costs – if it is looking like it is going to cost more than you expected, talk to your builder and see what can be done: are there any elements that are driving the price up? There may be parts of the build that the builder you have selected is not used to doing and may be sub-contracting parts out, which is increasing the price. I’m not suggesting haggle with the builder over the price, but there is always a small adjustment to a design that can be done to help lower the cost.