Do
Approach a professional first to guide you through what is likely to be acceptable. Choosing the right person to advise and draw up your plans is as important as choosing the right builder.
Plan ahead, define your objectives and prepare a realistic time schedule.
Look at your finances. Consider your budget carefully and how much value your extension will add to your property.
Check that the builder has relevant insurance, past experience and is able to offer references, check these references carefully.
Make sure you know the difference between a Quotation (a firm price and what you can expect to pay) and an Estimate (much less specific)
Don’t
Pay for any work in advance.
Change your mind when work begins on site if at all possible. It may result in a hefty budget overspend.
Necessarily go for the lowest quotes. Busy builders may not be as competitive, references are far more important.
Add too many bedrooms and not enough bathrooms. A good rule of thumb is one bathroom for every two bedrooms, with an en-suite for every guest bedroom. Otherwise, end value may be affected.
Run into money problems. Make sure your extension is properly planned, designed and costed.
Finally…. with sufficient forethought you will have planned your extension so that it will both be a great source of satisfaction to everyone living in the house, and also add at least as much to the value of the property as has been spent on it, hopefully considerably more.