Alterations and extensions to listed buildings

Achieving the correct balance between protecting the special interest of a listed building and proposals for alterations and extensions is a tricky task. Some buildings are more likely to be able to accommodate change than others. The extent to which a building can sustain alteration or extension will depend on building type and significance and the merit of the new work. Some buildings may be sensitive to even slight alterations whilst others may be less so.

The grade at which a building is listed is a material consideration in this balancing act, however, it is not a reliable guide to the sensitivity of a building to alteration or extension. Secondly, the acceptability of proposals will also depend on what work has already been undertaken to the building - the cumulative harm of successive small schemes can be detrimental to listed buildings and the nibbling away of special interest must be resisted. Where an addition is to be made, particular consideration should be given to the junction, with the aim that the new should accommodate the old'

Cleaning and repointing external walls

Cleaning the exterior surfaces of a building can have a serious and adverse impact on a building not only through altering its character and appearance, but also though damaging the surface and destroying detail. The SPAB strongly advocates the 'do nothing' approach where the building merely looks its age. Cleaning a building for the sake of it looking new is at odds with the SPAB approach and building conservation philosophy more generally.

A surface which is discoloured and worn by the elements tells a story of the buildings survival and age and this patina contributes to a buildings special interest. In some cases the shadowing that occurs on a building though weathering serves to accentuate architectural detail that is lost when cleaning is undertaken. Cleaning stone or brickwork often results in a monolithic and flat appearance and can be especially aesthetically jarring if only one house in a terrace is cleaned. In such a case not only has the appearance of the single dwelling been altered but the integrity of a homogeneous group of buildings has also been diminished.

Perhaps more important than the aesthetic consideration is the damage that can arise from cleaning a building. All cleaning methods can cause damage if incorrectly applied. Cleaning with water and a non-ferrous brush is the simplest method though this can lead to the over saturation of the building fabric. Other methods include abrasive and chemical cleaning and poultices which can destroy stone surfaces, remove the protective fireskin on brickwork and terracotta walls and destroy architectural detail. The SPAB will only support cleaning where it can be robustly demonstrated that the accretion of deposits or paints on a buildings surface is causing damage which is greater than the damage cleaning can cause.

Repair and replacement of plaster, render and paintwork

Historic plaster is an interesting feature of old buildings and should not be removed merely to expose the stonework, brick or timber-framed structure beneath that was never intended to be seen. Nor should plaster or render be removed because it looks old. Unless plaster or render is failing and that failure is causing damage to the building and can cannot be repaired in situ, it should be retained. Not only is the 'do nothing' approach the most economically effective, but historic surfaces have a unique patina which has formed over time. This patina is integral to the character and interest of historic buildings and should be respected.

Where it can be demonstrated that render or plaster does have to be replaced then traditional lime-based render should be used (unless the original render is itself cementitious). Due to its impermeability cement render is incompatible with buildings constructed out of traditional materials as it prevents any moisture within the wall from evaporating. Over time this trapped moisture will cause the wall surface to deteriorate and spall beneath the render. The use of cement based renders is highly unlikely to be acceptable at application stage.

Other historic renders such as stucco and Roman cement should be replaced and repaired like for like as they have a distinct appearance; they create smooth surfaces and allow for sharp edges often used to imitate ashlar stonework.

When re-rendering it is crucial that architectural decorative features are not obscured or concealed under the new render. Stuccoed elevations will often imitate mock jointing, rustication or plaster elements such as cornices. These should be retained or carefully copied using the traditional techniques to do so. Similarly, decorative plaster details such as pargetting and scraffito should not be removed or concealed under new render. The historic and architectural interest of both of these decorative techniques are such that any proposals to do so are likely to be strongly resisted.

In some circumstances the painting or repainting of a listed building will require consent, namely when the proposals would have an adverse impact on the special interest of the building by altering its character or appearance. As with render, cement based or or the impermeable or gloss paints should not be used on traditional buildings due to their impermeability. The correct finish for historic renders and plasters in lime-wash.

Where inappropriate renders or paints have been applied to the exterior of a building expert advice should be sought before any works are undertaken to remove it as doing so could cause catastrophic damage to the surface of the building.

It is important to note that whilst repainting with lead-based paints may be historically correct this activity is now restricted to Grade II* and Grade I listed buildings and must be referred to Historic England.

Repair and replacement of doors and windows

Door and window openings establish the character of an elevation; they should not generally be altered in their proportions or details, especially where they are an intentional element of the building's design.

If a door must be replaced an off the peg modern door is not likely to be acceptable in a listed building and every effort should be made for the new door to be sympathetic to the interest of the building. In addition, doorcases and door furniture (hinges, knockers, handles etc) should be retained wherever possible as these details are indicators of age and function.

Window types vary according to region and building tradition and as such can be important features of an historic building; within the general window types such as sash or casement, there is a diversity of details according to date, function and region.

As a rule, windows in listed buildings should be repaired - often replacement occurs where repair is a better outcome for the building ans sometimes more economic - or replaced like for like where repair is not viable. Where a building has had it's original windows replaced, there is often a desire to return to the original fenestration pattern It should be noted though, that later window may have interest in their own right and speak to the changes in fashion or fortune of past inhabitants. Where the newer windows are obviously inappropriate, replacement windows should be appropriate to the date of the building. It is worth noting that the depth to which window frames are recessed within a wall is a varying historical feature of importance and greatly affects the character of a building: this too should be respected. when considering replacement.

It is usually near-impossible to install double-glazed windows in existing frames or to replicate existing frames with new sealed units without making noticeable changes to the profiles of glazing bars, styles, and rails. The new glass in such units may also significantly alter the appearance of the window and the replacement of historic glass is rarely acceptable in listed buildings. If the desire for double-glazed units is being driven by energy performance, there are many solutions which can provide similar efficiency benefit. [LINK TO tech advice on retrofitting & energy efficiency briefing]

Alterations to roofs

The roof is often the most dominant feature of a building and every effort should be made to retain its original structure, pitch, cladding material and ornament. Whilst a roof has a very important structural and protective role to play, they often have architecturally interesting qualities as well. For example the pattern and coursing of different stone and slate tiles varies regionally and nationally and the difference in roofing materials from clay to slate and thatch to stone is a distinguishing feature of different building types and areas of the country. Where traditional and/or local roofing materials exist their retention should be priority. Should the roof need to be stripped as much of the original roofing material should be re-used.

The addition of new dormers and rooflights to an historic roof must be approached carefully. The existing historic roof structure should not be damaged in order to facilitate a either option and is likely to only be acceptable where they are least visible and do not compromise the special interest of the building. It is important to consider the impact of new dormers and rooflights on historic terraces: should proposals for either disrupt the symmetry design, it is likely they would harm the special architectural interest of the group.

Alterations to structure and plan form

The plan form of a building is often one of its most important characteristics and the internal partitions, staircases and other features which create historic plan form are integral to retaining that significance. Proposals to reconfigure or remove internal arrangements including forming new openings, removing internal partitions or blocking staircases will have to be well justified and will be subject to the same consideration of impact on special interest as for externally visible alterations.

In a similar vein, new partitions in historic buildings should be kept to a minimum and should not cut through mouldings or decorative plaster, but be scribed around them so as to be reversible in the future.

One sadly popular alteration people make to plan form (other than than the sub-divsion or opening up of rooms) is the removal of chimney breasts. There is no excuse for their removal if this is simply to rationalise a room or if the chimney is redundant and is likely to be unacceptable, not least because of the impact on the structural stability of the building.

Alterations to internal features

The internal features one finds in a historic building, including staircases, joinery such as window shutters, doors and doorcases, panelling, decorative mouldings and ceilings, stucco-work, chimneypieces and wall decorations and paintings all come together to form the special interest of a building and may be its most significant feature. The removal or erasure of any of these features is unlikely to be acceptable.

Even where buildings are not highly decorative they likely contain historic joinery and plasterwork which speak of the materials and construction of the time the building was built. All original plasterwork should be repaired and retained wherever possible and great care should be taken where new services are proposed. Chasing in wiring to historic plaster is unlikely to be acceptable especially where it is highly decorative or painted.

Repair or replacement of floors

The floors of old buildings are often worn, discoloured, and out of true level. Yet these imperfections can make their own important contribution to the interest, beauty and historic value of a building. Floor surfaces are all too often disregarded because of their function or because of their modest value: it is not only marble floors that are important. All types of paving such as stone flags, and pitched cobbles, old brick floors, early concrete, lime ash, and plaster floors, should be respected. This also applies to old boarded floors, especially those with early wide oak or elm boards.

The SPAB believes that it is the imperfections and wear on historic floor surfaces that make them special, therefore, we advise that they only be lifted in rare and well justified circumstances. If any floor replacement needs to occur the timber, flag or tile should be of the same material, width and thickness as those they are replacing.

Proposals for floor strengthening often come about as part of a proposals for a change of use or refurbishment. This is considered to be a major intervention into a listed building. The standards applied to new buildings are at odds with historic buildings and fabric and imposing those same standards usually results in an unacceptable loss of historic fabric and harm to the building. Limited strengthening may be possible if there is demonstrable need and the intervention is kept to the absolute minimum. The structural equilibrium of historic buildings is a finely balanced thing and introducing modern structural materials can have devastating consequences. Repairs for strengthening should be carried out using traditional materials and methods: should modern material be proposed then this will require robust evidence and justification.

Basement alterations to provide residential use

In many cases the basement may be one of the few parts of the building that has not seen a great deal of alteration and, as such, it provides valuable evidence of the building’s original plan form, history and development. Basements may also be one of the last areas to retain any surviving original finishes, which should therefore be protected and retained rather than destroyed.

In The London Terraced House (1756), Isaac Ware writes on the subject of basements and vaults: ‘the lower story in these common houses in London is sunk entirely underground, for which reason it is damp, unwholesome and uncomfortable.’ (p.346). This is a description of the very necessary nature of these spaces as they are designed to allow for the passage of moisture through the building. The conversion of damp vaults and basements for residential use is difficult to achieve without an inappropriate level of intervention into a building, and in most cases the SPAB would not support such proposals. Even where vaults are relatively damp free, issues of natural light and outlook are likely to be at play and preclude basements from being fit spaces for core living purposes. These spaces were designed to be ancillary to the habitable spaces of a house and to change the nature of that space for human habitation is likely to go against the SPAB approach.

Where basements and vaults are too damp to fulfill their purpose as ancillary storage space we accept that damp can be managed with limited intervention. The simplest and least intrusive approach is to use a lime plaster on internal wall surface and a limecrete product to the floor, provided that any features of special interest are retained and not harmed in doing so. However, it is also important to consider that excessive damp is often a symptom of problems of water ingress and rather than treating the symptom the cause should be full understood and the appropriate action taken.

The SPAB does not support damp treatment methods such as chemical injection or cementitious tanking. These impermeable 'solutions' are not compatible with traditional, permeable building materials as they do not allow for the passage of moisture through the building. It is likely that moisture will become trapped in the building's fabric or be displaced elsewhere.

Introduction of services to historic buildings

The introduction of new services to historic buildings can, if not well designed and carefully considered, be detrimental to the character, appearance and even structure of an historic building. Long runs of surface wiring and gas and central heating piping should be avoided where visible externally, unless chasing-in would destroy historic fabric.

Hastily carried out or ill-conceived work to install new services into the interior spaces of historic buildings can result in and a feeling that the building is no longer 'old'. For example ceiling downlights and very white lumieres in historic buildings result in a feeling of clinical newness which is at complete odds with the atmosphere of old buildings. The same follows for surface mounted modern trunking to conceal wiring. The cutting of floor joists for new services and wiring should be kept to an absolute minimum and will only be acceptable if it can be demonstrated that it is totally necessary. Any early examples of sound-deadening or fire-proofing between the joists should be preserved

There are some standard external fixtures that require listed building consent when they affect the special interest of a listed building. These include satellite dishes, meter boxes, burglar alarms, security and other floodlighting, video cameras, and central heating and other flues, both standard and balanced. Only visually unobtrusive positions for such fixtures should be sought. The principle should follows for unlisted but historic buildings.

Alterations to shopfronts

Original and historic shopfronts add a great deal of character and architectural interest to individual buildings and to the wider streetscape. Wherever shopfronts of architectural and historic interest remain they should be repaired and retained rather than replaced. Features of shopfronts from the 18th and 19th century (and perhaps even the 20th century) may be concealed under more contemporary shopfronts and so where works to modern shopfronts in historic buildings are proposed, the potential for survival of features should be borne in mind and works undertaken carefully. Where a new shopfront is required it should be designed in sympathy with the rest of the elevation it is to occupy.


In judging the suitability of an extension to a listed building it is essential that the the elements of the building which contribute to its special interest are understood. This pertains to the obvious elements like external and interval features such as windows and doors, staircases or decorative schemes, but also the less obvious, such as plan form (layout) of the building and the hierarchy of spaces within the building, as well as any archaeology on the site. Any new extension must take these factors into consideration and be sympathetic to them.

Basement extensions

The issue of excavating basements under London townhouses has been a hot topic in conservation circles in London for some time and we expect this trend will become more widespread. The SPAB is concerned that excavating beneath a property to create a new or extended basement will have a profound impact upon the historic fabric and character of the heritage asset and also on the legibility of the building’s development and hierarchy of spaces, even though the changes will not be very visible other than from within the basement itself.

The SPAB also takes the view that traditional buildings have an intimate connection with the land on which they are constructed and the relationship between the building and the ground below its foundations should not be lightly dismissed. Excavating beneath the ground floor of the building breaks this intrinsic and important connection with the land and would result in a building that is no longer in its original context, and should therefore be firmly resisted.

There are, of course, also a whole host of related and very serious technical concerns regarding the physical impact on the neighboring properties. It is extremely difficult to predict accurately how such a large and invasive excavation might affect the water table and the movement of water within the ground around a property, but it seems likely that there is at least some level of risk to the building and to any neighboring structures.

The SPAB is also concerned about the introduction of additional light wells, as these will inevitably change the appearance of the outside of the building and will have an adverse impact on its setting due to the light that will wash upwards across the elevations when the lights are used in the basement in the evening.

The Royal Borough of Kensington and Chelsea has adopted a new local planning policy (CL7) which indicates that basement developments should not extend under listed properties. The SPAB wholeheartedly endorses this policy and hopes to see a similar approach taken in the other London boroughs and authorities nationally in due course.

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