It is important to consider the reasons for your 'improvement'. For example, is it more important to increase your property's value or to make your home more comfortable? There is usually a trade-off between these two objectives. Also, bear in mind that if you move soon after making an expensive improvement, you may not recover the construction costs.
Improvements that add space, add most value.
Extensions, especially those where you can't see the join and which follow the style and scale of the existing house are best.
Conservatories can provide an instant space-making solution. The best ones unite house and garden, without substantially reducing the available garden - and look an integral part of the house.
Converting the loft is usually simpler than building an extension. A well-conceived design can create extra space and increase the value of your home by up to 10 per cent. Victorian houses probably offer most scope - the roof spaces are usually larger and the timbers may not need to be reinforced.
Space-making improvements may need Planning Permission and those that don't will probably need Building Regulations approval. You may have certain 'permitted development' rights - please check with your local authority.
Installing central heating is usually a worthwhile improvement, as are sympathetic building restoration and adding a garage.
Double glazing is a popular improvement but, unless the window frames need replacing, it can be one of the least cost-effective. The capital cost could take 15 years to recoup through energy savings, although the comfort and quiet gains are less quantifiable. Costs vary considerably.
You could also consider installing home security, refitting the kitchen, refitting the bathroom and interior decoration.
The market value of a home is influenced most by its location. All properties and areas have an optimum value and there is a limit to how much an improvement can increase the price. In practice, the reality is that your house needs to look much like the one next door - but better.