Building restoration describes the process of the renewal and refurbishment of the fabric of a building. The phrase covers a wide span of activities, from the cleaning of the interior or exterior of a building – such as is currently underway at St Paul’s Cathedral in London – to the rebuilding of damaged or derelict buildings, such as the restoration of the Windsor Great Hall in Windsor Castle after a destructive fire in 1992. The 1985–1989 removal of 38 layers of paint and the cleaning and repair of the exterior sandstone walls of the White House in the U.S. are an example of building restoration.
Buildings are structures which have, from time to time, particular purposes. They require ongoing maintenance to prevent them falling into disrepair as a result of the ravages of time and use. Building restoration can be thought of as that set of activities which are greater than year-to-year maintenance, but which by retaining the building are less than a demolition and the construction of a new building.
The scope of restoration depends upon the need, and other circumstances, such as the status of the building, and the affordability of the work required. There are three main types of restoration:
- Building cleaning – most especially cleaning the external facade of a building, and typically needed in cities that have suffered from smoke pollution. Many granite, sandstone, and limestone buildings in the UK, for example, have for most of their existence been black in colour owing to smoke and smog. Many, in turn, have been cleaned after air pollution legislation diminished the incidence of airborne particulate matter. Any building that has suffered from fire and/or water damage, needs to be restored as well. Fire and water restoration specialists can help speed repairs, whether for individual homeowners or for the largest of institutions. 
- Major repair – especially to stonework affected by acid rain and other pollutants, and which has weathered or decayed to a structurally unsound or aesthetically displeasing condition.
- Rebuilding to replace severely damaged or missing parts of a building. Here, in all cases, a balance is to be struck between recreation of the original building using materials and techniques similar to the original construction, as happened at very great expense at Windsor Castle; and the use of more modern techniques and materials.
Not all building restoration seeks to follow the original design of the building. It is reasonably commonplace for the shell of a building – its external walls – to be retained whilst an entirely new building is constructed within. This approach is also referred to as adaptive reuse.
Caspar Ott Cabin from 1837 in Deerfield, IL restored by Bob Przewlocki.
Although techniques of restoration are improving, the action of cleaning or repairing buildings can, with hindsight, be seen to cause problems that at the time were unforeseen. A good example is the unrestrained use of sandblasting to clean smog deposits from soft-stoned buildings – a technique employed in the UK in the 1960s and 1970s – which has damaged the external faces of stonework to the extent that in some cases, later, the stonework has needed to be replaced. Contemporary building codes recognise such problems, and (it is to be hoped) mitigate poor outcomes.