According to the Ideal Home Magazine and the Royal Institution of Chartered Surveyors........

"Adding a conservatory can raise your house's value by 4% to 5% according to the RICS and is a relatively simple and cost-effective way of increasing your space. Choose a design that complements the style of your home and make sure it doesn't dominate your garden. Also think about its position: a south-facing conservatory needs ventilation and shade, while a north-facing room requires good heating. Check with your local planning department whether you need permission. If you're within your home's permitted development allowance (your local planning department can advise on this) and your conservatory is separated from your house by patio doors, you may be exempt."