The trick to building an extension is to ensure the project progresses on time and to plan. According to chartered surveyor Kevin Hollingsworth the single biggest thing to avoid is changing your mind halfway through the job. “The biggest mistake is changing from the original quote so much that it’s a different job,” he says. “That’s where it all goes wrong financially. Put all your thought in before you get prices.”

Wishlist
First off, decide what you want to achieve. Stephen Parker, an architect who specialises in home extensions, advises clients to begin by imagining what they would do if money were no object. "Create a wishlist of what you want. Don't be too restricted with thoughts of cost," he says. "Separately, come up with your budget."

An architect should then be able to devise a creative solution. “Extensions could add a small area but reclaim space that wasn’t used very well,” says Parker.

The most common type of extension is a single-storey rear extension but even within this there are infinite variables. “There are hundreds of decisions to be made,” says Parker. “Floors, walls, roof, windows, doors, joinery, roof lights, drainage, finishes. You need to be able to make informed decisions.”

Money
Your budget will be driven on the one hand by what you can afford and on the other by the design and quality of your desired extension. Don't forget to include professional fees and decorating costs in your total budget, as well as the cost of construction.

To get the best price for the job, Hollingsworth advises putting your extension out to tender and getting several quotes to compare. He says it’s important to make sure every contractor is given the same information so they can quote on an equal basis and you can make a fair comparison.

At this stage, the costs are within your control, unlike further down the line. “If a contractor comes in for a job and does that job, it’s very difficult for them to change the price,” says Hollingsworth. “When you have a contractor on site and you want extra work, it’s not competitive because you’re locked in and you don’t know if you’re getting good value for money.”

Parker suggests keeping a contingency fund to cover unforeseeable costs, such as extra work required if drains are uncovered when digging foundations. Older houses may need bigger contingency funds as they are less predictable.

Experts
If you're feeling flush you can engage a project manager to carry out all stages of your extension, from the preliminary drawings to the snag list. They will tender your work, draw up contracts, make sure regulatory requirements are met and manage the construction process. While you may prefer to do some of the work yourself, Hollingsworth advises that, at the very least, you should engage professionals to sketch plans with specifications, carry out key stage inspections and for the issuance of certificates of compliance.

Professionals are best recommended by other professionals or by word of mouth, and should have references. View their previous work and make sure you can get along with them for the duration of the project.

Timeframe
How long your extension will take to complete will depend both on the complexity of the project and any unforeseen issues that may arise. You should have a contract with your builder that specifies a start and an end date. However, should the project overrun without good reason your contract may contain a provision for you to claim liquidated and ascertained damages, a monetary sum for each day or week the project is behind. You must be able to show a genuine loss in order to claim.

Permission
If your extension is going to increase your original house by more than 40sq m, you'll need to seek planning permission. Many home extensions are classified as exempted developments and your architect or surveyor will advise on whether your plans fall into this category.

Extensions of more than 40sq m are now also subject to new building regulations. This requires an assigned certifier such as a registered building surveyor, architect or civil engineer to sign off extensions at key stages throughout the process. If, in the future you decide to sell up, proof of certification will be required as part of the house sale.

However, Hollingsworth advises that you ask your professional to undertake inspections even if it’s not required by law, both for peace of mind and to keep your builders on their toes. The key stages he recommends for inspection are foundations, damp-proof coursing and membrane, insulation and roof.

Protection
Although you will have the opportunity to create a snag list at the end, Parker says it's not good practice to rely on snag lists to sort out everything.

Most contracts contain a provision for defects liability protection, which sets out a period of time during which faults can be discovered and repaired for no extra charge. Make sure you also get any electrical test certificates and gas certificates of performance.

• Building your extension will have an impact on your neighbours, from increased noise and activity to skips and machinery on the street. You may need their permission for certain things so it's a good idea to talk to them in advance.
• Think about whether you want to live in your house while the work is being carried out. Can you really put up with living on a building site for several months? Don't forget to budget for alternative accommodation if necessary.
• Just because your extension is below 40sq m doesn't mean you can do what you like. Planning permission is required if you wish to reduce your garden to less than 25sq m or build an extension that exceeds the height of your house.
• Think about how your extension will affect your current rooms. If you transform an exterior room to an interior room, then you might need to add roof lights to keep it bright. An extension could make your existing rooms warmer by adding insulation.
• An extension is likely to affect the value of your house, which in turn may affect your local property tax band. While your current LPT rate is fixed until 2016, you may find yourself in a different situation when tax is next assessed in November 2016.