Selling land that has been developed to raise its value is subject to taxation. A straightforward land or property sale would normally incur a Capital Gains Tax (CGT) charge. Gain is calculated as sale price, less purchase cost, and any qualifying expenditure, and less incidental costs of purchase or sale.
If the asset is held by an individual, the gain will then normally be taxable at 20% to the extent that it falls within the basic rate income tax threshold (between £12,571 and £50,270). The higher 28% CGT rate which applies to disposals of residential property will not apply to a disposal of bare land, even if it already has planning permission for residential property to be built, but would apply if what you are selling is already residential property.