Whilst this is one of the most important considerations when thinking of extending your home, it is also the most difficult subject to cover precisely.

To help you consider this subject in more detail, we have summarized the main cost variables below under design factors, building factors and other factors. All of these will have an impact on the overall cost of your project.

Remember to add the cost of any design/architects fees, council charges or other professionals such as structural engineers and party wall surveyors to your total.

A. Design Factors

The various aspects of the design of your proposal will play a major part in determining the cost of the project. Be sure that before you agree to a final design, you have a good idea of the likely cost as it can be expensive and time consuming to change your plans once they have been agreed and submitted to the council for approval.


Clearly, the size of the extension will have an effect on the overall cost. Generally the bigger the extension, the more expensive it will be as more materials, labour and plant are required to build it. However, be aware that two extensions of exactly the same size can be vastly different in cost due the effect of the other factors listed below.

Design Style

Modern design
Remember that ultra modern can also mean ultra expensive!

The final design style of your extension will have a significant affect on the overall cost - generally, the more complex the design, the more expensive it will be. For example, a curved wall with large amounts of glass will cost more than a straight wall with a single window or door. If you are creating large open spaces in the extension or as part of the redesign of the existing house, remember that extensive structural steel work is expensive.

We would be keen to encourage you to create a beautiful building, but be aware of the potential cost impact of any design decisions that you make.

Specification and Materials

Your choice of materials and specification will have a major impact on the overall cost of the works.

Some items, you will have little or no choice on such as bricks and roof tile types as these will normally be expected to match the existing property. Plain, hand made clay roof tiles can be 3 or 4 times the cost of modern interlocking concrete roof tiles and the same kind of variation can apply to bricks and many other building materials.

It is not uncommon to find that the cost of the basic shell of the building is only half of the overall finished cost. Your choice of fixtures, fittings and finishes will have a significant influence on the final cost of your extension.

Supply and fitting kitchen units and appliances can vary from £1,500 to £50,000. Sanitaryware such as showers, basins and baths vary hugely in cost.

When budgeting for your project, make sure that you allow sums for all the items of work including the finishes as these will all form part of your decision as to whether extending is an affordable option for you.

Shower type 1shower type 2
Fixtures and fittings can vary enormously in cost


The use of the new rooms will affect the cost as some rooms are more expensive to build than others. Kitchens and bathrooms, for example, cost more than bedrooms and living rooms per square metre due to the fitting costs and additional services required.


Example of a bad quality plan
Poor quality drawings such as this can lead to builders’ claims or work having to be redone.

Your choice of architect or designer will have an effect on the overall project cost. Some designers will cost more than others although be aware that a well thought out and specified design could save you money in the long run by avoiding misunderstandings at the building stage.

At myhome mygarden, we believe that our design fees are competitive and offer excellent value for money. We always work on a fixed fee basis so you know exactly how much the design work for your project will cost you.

B. Building Factors

If you were to give the same set of plans to twenty separate builders, it is unlikely that any two would provide you with exactly the same figure. It is not uncommon to find a vast difference between the lowest and the highest quotations, leaving you as the homeowner with no idea of what the true value is and who to choose to carry out your work.

Below we look at the various factors at the building stage that will affect the overall cost of your project.


Mini digger

Access to the site can affect the building cost as land locked gardens (i.e. where there is no access except through the house itself) will lead to the work being more labour intensive.

In the last decade, the building process has become more and more mechanized and if the builder can not complete the work using equipment such as mini-diggers, they are likely to charge a premium for this.

Choice of Builder

Your choice of builder will undoubtedly make a difference to any quotation you receive. Builders work in different ways with some carrying out one job at a time and others carrying out many projects simultaneously. Some will have higher overhead costs (i.e. office, yard, vehicles, etc) and will need to allow for these in their quotation.


Builders will take differing views on the level of profit they will expect. Some are happy to earn a daily wage and will treat any additional profit they make as a bonus whilst others will want a good return in addition to a daily rate. Generally successful and busy builders with good reputations know that they do not have to charge rock bottom prices to get work in and will probably be more expensive.

At myhome mygarden, we carry out a thorough vetting process for the builders and tradesmen we use and we know that ridiculously cheap quotations generally lead to trouble.

Temporary Works

With certain projects you will have to consider the option of carrying out temporary works. For example, extensions that involve opening up the existing roof may require a temporary roof in order to minimize the risk of rainwater damage. You may require temporary storage space creating in your garden to store personal belongings whilst works are carried out. Always check what your builder has allowed for in his quotation in order to make sure you are comparing 'apples with apples'.

The Unexpected

Even the best builders in the land are not fortune tellers and finding unexpected additional costs once works have commenced is a regular occurrence on building projects.

Old well
Many Victorian properties were supplied with water from a well which may still be located in the area of your proposed extension.

Groundworks such as foundations and drainage works, in particular, are the most common areas for this due to the difficulty in assessing conditions on the site. You may discover all sorts of things including wells, poor ground conditions, air-raid shelters, damaged drains, water pipes or electric cables in the area where works are being carried out.

Other areas of work such as rotten or woodworm infected timbers, poor quality plumbing or electrics may also be discovered once works have commenced. The older the property, generally the more chance there is of finding the unexpected.

We would advise you to allow a contingency sum in your budget to cover the unknown.

Human Error/Misunderstanding

Whilst this may sound minor, many arguments between the customer and builder relate to the original cost of work and what was included in the builder’s quotation. The builder may have genuinely missed an item of work out when preparing his quote which other builders have allowed for and you assumed was included.

Make sure that all the work is clearly defined in order to avoid this as you will encounter problems if you choose a builder that is half the price of all the others because he has not included some of the work. This can normally result in continual requests for additional payments or the builder walking off site or even going bust.

C. Other Factors

There are some influences on the cost of the work that do not fall into either of the above categories and these are summarized below:


Traffic problems
Building in city centres will often cost more due to traffic and parking problems.

The location of the property in the country will affect the overall cost as labour costs vary between the north and the south. As property is cheaper to buy or rent in the north, tradesmen can charge lower rates for their services. The Royal Institute of Chartered Surveyors (RICS) provide a Property Makeover Price Guide which, using a specific example of a 3m x 3m single storey extension shows that it would cost approximately £23,940 in Greater London compared with £19,320 in the North West.

You may also find that properties in city centres are more expensive to extend due to the additional time and cost of deliveries, getting to work and parking.


In recent years there has been a glut of quality EU tradesmen but many of these are now returning to their native countries and this has reduced the level of competition. The RICS Property Makeover Cost Guide states that the cost of improvement work has increased by 20% in the last two years as a result of this and rising costs of transport and raw materials.

Be aware that builders quotations will only be valid for a certain period of time and the cost of the project may rise if you decide to go ahead after the validity date.

Interest Rates/Inflation

Many people fund their project by extending their mortgage so any change in the interest rate may have an impact on the amount you repay.


Some design services and builders are not VAT registered as their turnover is lower than the required limit. Businesses with annual sales below £67,000 (2008-2009) do not have to register for (and therefore charge) VAT, but they may choose to do so voluntarily. If they are not registered, the price you pay for their goods or services may be cheaper than if you bought the same goods or services from a VAT-registered supplier.

However, be aware that £67,000 in building turnover is not a high level and builders working below this are likely to be small jobbing builders.

Some works such as works for disabled persons are zero rated for VAT so you will be able to claim any VAT incurred back.


Some or all of the above items will have an affect on how much your project will cost you and they demonstrate the importance of getting expert advice and employing experienced people to carry out your project for you. Get advice at an early stage.

my home my gardenHow can we help?

myhome mygarden can provide a comprehensive Feasibility Study for any home extension that you are considering which is an excellent way of finding out how much a proposal will cost.

See the Feasibility Study section of our site for more information and a sample report and plans.

If you decide to use myhome mygarden for your extension project, our system is set up to minimize misunderstandings and building costs are tightly controlled under our Project Management system.

Call us Free on 0800 3800 141 to find out more.