People in Glass Houses

Designing conservatories for Historic Houses

David McDonald


  (Vale Garden Houses)

How many times do we see advertisements which urge us to 'extend our homes tastefully and economically' by buying some brand or other of conservatory? For many home owners it clearly is an attractive proposition and if well designed, a light, timber and glass structure may present the most sensitive means of extending an historic house, allowing the original form of the building to be seen clearly. However, a poor design can look ugly and out of place and whether a conservatory might be an appropriate addition to an historic building needs careful consideration.

At the Royal Borough of Kensington and Chelsea, conservation officers deal with a considerable number of applications for conservatories every year both on listed buildings and unlisted buildings in conservation areas. This is perhaps not surprising in a borough which includes some of the wealthiest areas of London, but the fact that it also has some of the highest residential densities in the UK makes any extension potentially problematic, affecting the privacy of neighbours and the character of their surroundings even before any historic building issues are considered. Kensington and Chelsea is probably the only local authority in London which has a specific policy on conservatories in its statutory planning policy, or 'Unitary Development Plan'. This article deals with conservatories primarily in an urban setting where the opportunities for conservatories are most restricted, though generally the same principles will also apply to suburban or rural areas.


The earliest known conservatories date from the 17th century. The first in Britain is believed to have been constructed in the Oxford Botanic Gardens, followed soon after by another example in the Chelsea Physic Garden. In the 18th century, the orangery became a fashionable addition to the English country house. At Kensington Palace in 1704 Queen Anne commissioned a Baroque design, attributed to Hawksmoor for a free-standing orangery in the grounds. However, it was not until the early 19th century that conservatories came within the reach of private individuals. Glass houses of all kinds became popular in the mid-19th century as improvements in technology led to cheaper glass and cast iron and enabled larger sheets of glass to be produced. Paxton's Crystal Palace, constructed for the 1851 Great Exhibition, provided a dramatic advertisement for glazed structures and was a major influence on the popularity of conservatories. They reached their zenith in the late Victorian and Edwardian era, but by the end of the First World War they were no longer in fashion. In the last 30 years there has been a resurgence in their popularity and hence the number of firms producing a variety of proprietary and bespoke designs of differing quality and cost.

The original purpose of a conservatory was, as in the current dictionary definition, 'a greenhouse for tender plants'. In the late 19th and early 20th centuries they were also used for entertaining, but maintained their role as a natural transition between house and garden. Gardening writer Shirley Hibberd (1825-1890) wrote: 'A conservatory should be a garden under glass and a place for frequent resort and agreeable assemblage at all seasons and especially at times of festivity'. However, today there are many uses to which conservatories may be put, such as living or dining rooms. In other words, building a conservatory is just another way of extending a house, and this should be borne in mind when considering its design.


It is the original use as a form of greenhouse or garden room that should perhaps inform our philosophy of design. Its success as an addition to a house often depends on its proximity to the garden and its appearance as a light-weight addition, clearly subservient to the parent building and in an appropriate style. The Victorians were conscious of this. Mrs Beeton in her Dictionary of Everyday Gardening (1896) advised that the architectural style of a conservatory should be in harmony with that of the house. There are three main factors in considering how a conservatory might be designed: its location in relation to the house and garden, its size and its detailing.

  JUST LANDED. Although ideally located for a first floor flat, the roof-top conservatory is an alien feature in this otherwise typical Kensington street scene. Without strict planning controls, the townscape of neighbourhoods such as this would be dominated by roof-top conservatories.

When considering the introduction of a conservatory, location is obviously a fundamental issue. The most natural location would usually be at garden level to the rear of a property (the basement level in some cases). Victorian and Edwardian conservatories are sometimes found at the front of a house which is set in its own grounds, away from passing traffic, but hardly ever in front of a town house, and a new conservatory would normally look out of place in this location. In urban streets there may also be a problem locating a conservatory at the side of a property as the views between houses are almost always important to the character of the street. If placed on a roof or on top of an existing extension a conservatory can look completely bizarre, having lost its relationship to the garden. Where the prevalent pattern at the rear of Georgian and Victorian terraced houses is that of repeated light-wells and rear extensions (or 'closet wings'), it is most common to place the conservatory in the light-well between the extensions. Setting back the rear building line of the conservatory from that of the adjacent extension also helps to maintain the rhythm of original extensions and limits intrusion into the garden itself.

An important consideration is how the conservatory is linked to the parent building. Where a building is listed, protection includes the whole of the interior, not just the exterior, and the cellular nature of historic buildings is a key element of their character. The local authority's conservation officer is unlikely to allow a new conservatory to be merged with an existing room by the demolition of a whole wall as this would entail considerable removal of original fabric and alter the room's proportions and character. Much less intervention is necessary if an existing opening is used. Ideally this would be a door, but often it may be possible to drop the sill of an existing window opening to create a doorway. If the window is to be removed, consider reusing it elsewhere on the building rather than discarding original or historic fabric, particularly if other extensions and alterations are proposed at the same time. By minimising alterations in this way, the original building may continue to be 'read' through the conservatory.


The size of the proposed conservatory is also of crucial importance. At its most fundamental, all extensions must be clearly subordinate to the parent building if the character of the original architecture is to remain dominant, and should generally cover no more than half the width of the elevation.

When considering the footprint of a conservatory, the cellular structure of the house and the sense of hierarchy between front and rear rooms can be taken as a starting point. In most cases its floor area should be significantly less than that of the adjoining room to maintain the progression through the house from grand space to lesser spaces.

The plan-form should relate to that of the parent building and will often be dictated by the conservatory's location. Usually a square or oblong plan is adopted, but in some cases chamfering the corners may soften the appearance of a large structure and a polygonal form may relate better to existing features.

Two storey or double height conservatories are rarely appropriate additions to listed buildings as their overall scale can look out of place, and they may obscure relatively large areas of the elevation of the building. The same may be true of over-elaborate roof structures.


A glance at manufacturers' catalogues at the quality end of the market shows a preponderance of 'traditional designs' These are often based on Victorian and Edwardian styles and consequently may appear very ornate and fussy in appearance. On Georgian or early to mid-19th century houses these styles are likely to clash with the rather plain and restrained appearance of rear elevations. It is better to reflect existing character by simplifying the design. More elaborate designs are best suited to buildings of the Domestic Revival of the late 19th and early 20th centuries. Obviously a quite different approach needs to be taken with buildings of the Modern Movement and contemporary structural patterns and glazing might be utilised.

A potentially successful design may be let down by poor detailing. This may show on the solid lower sections of the conservatory where brick construction may give an unduly heavy or incongruous appearance. Timber panels are often crudely constructed with applied mouldings to imitate traditional panelling, and equal attention needs to be paid to the framing of the glazing which should be lightweight and delicately detailed. Double-glazed units with false glazing bars look false, particularly from the inside. Similarly, the appearance of the roof structure may be compromised by the use of wide aluminium cover-strips.

  Leaded lights set in green painted joinery help to relate this conservatory to the original 1930s architecture of the house and its garden. (Vale Garden Houses)

The colour for the joinery of the conservatory may be chosen to match the existing woodwork of the house. However, as this colour is usually white, one interesting alternative is to paint the conservatory dark green to reduce its visual impact.


In conservatory design over the years, glass has been used relatively consistently although the type and quality has changed considerably as manufacturing processes have become more efficient. On domestic conservatories timber was the predominant framing material in the 19th and early 20th century with cast iron generally confined to the strengthening brackets, cresting and finials. This is in contrast with larger palm houses and winter gardens which tended to be predominantly iron structures. For the construction of traditional conservatories attached to listed houses today, timber is still the preferred material as it combines strength with potentially fine detailing. Aluminium and upvc as well as not being traditional materials and with a quite different appearance to timber, require larger cross-sections and inevitably much cruder detailing. Narrow sectioned steel (such as w20) may be more appropriate for conservatories attached to buildings of the modern movement and later.

There are a few examples of effectively 'frameless' conservatories which are almost entirely constructed of glass. This uncompromisingly minimalist and transparent approach to design may, on rare occasions be acceptable on a house which is listed, but usually only in relatively concealed locations. They may be justified where there is minimal alteration to the fabric of the building and where they allow the original elevation to be seen through the glazed structure.


Under current building regulations a conservatory may be considered as an exempted structure if it meets the following criteria: it is a predominantly glazed structure; it is at ground level; it has a floor area of less than 30sq metres; it is separated from the main building (that is, accessed via a door rather than being an enlargement of an existing room). If these criteria are not met the conservatory would be treated as any other extension and would need to meet normal building regulation requirements for construction, including drainage, ventilation, thermal insulation, and fire protection. Thermal requirements in particular would be almost impossible to achieve with a predominantly glazed structure.

Recommended Reading
(Updated 2010)

  • English Heritage, London Terrace Houses 1660-1860: A Guide to Alterations and Extensions - Part 2, London, 1996
  • J Hix, The Glasshouse, Phaidon, London, 1974
  • M Woods & AS Warren, Glass Houses: A History of Greenhouses, Orangeries and Conservatories, Rizzoli, New York, 1988


This article is reproduced from The Building Conservation Directory, 2000


DAVID MACDONALD is a member of the Institute of Historic Building Conservation and is responsible for conservation and design at the Royal Borough of Kensington and Chelsea. The views expressed are his own and do not necessarily represent those of the local authority.

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