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Window sizes

• Introduction
• The historical background
• The current situation
• The future

Introduction

• The intention of this page is to dispel some of the myths surrounding window sizes and to provide some specific guidance.
• It is generally assumed that the provision of natural daylighting to habitable rooms in dwellings is a requirement of the current Building Regulations. This is not currently correct for England and Wales.
• There is a popular, and often quoted, rule-of-thumb that "The area of window(s) in a room should be at least 10% of the floor area of the room" but this appears to have very little basis in current law.
• The situation is different in Scotland where there are specific requirements relating to window sizes. These can be found on CHECK LINK of the Scottish Building Standards website. Those unfamiliar with the Scottish Building Standards should be aware that, in this context, "Apartment" means a room in a dwelling not used solely as a kitchen, store or utility room.

The historical background

The Building Regulations 1976

• Part K of these (now obsolete) Regulations dealt with open space, ventilation and height of rooms. It is worth noting that this heading did not include daylighting since the Regulation dealt, primarily, with the achievement of good ventilation to habitable rooms.
• Part K1 of these Regulations, dealing with the open space outside windows of habitable rooms, described, in paragraph 3(b), a window as having an inner plane which: "...has a width such that the product of that width and the window height equals one tenth of the floor area of the room containing the window."
• This paragraph would appear to be the origin of the "10% rule".
• The Building Regulations 1976 are not available on the internet.

The Building Regulations 1985/2000

• The 1985 edition of the regulations was radically different from the 1976 version. It was the first time that the Approved Document format was adopted.
• Ventilation was contained in Approved Document F and was greatly simplified. The "10% rule" above was omitted and the only reference to openings was that the requirement would be satisfied if there is: "for rapid ventilation, one or more ventilation openings with a total area of at least 1/20th of the floor area...........eg an openable window."
• This "1/20th" requirement for ventilation was still in the 2000 Edition of Approved Document F, in Section 1, Table 1.
• The 2006 Edition of Approved Document F, Section 1, makes no reference to openable windows as a means of providing adequate ventilation. The assumption is that all ventilation requirements must now be provided by mechanical extract or passive ventilation systems.

The Housing Act 1985 and later amendments

• This was published in the same year as Approved Documents were introduced into the Building Regulations. The Housing Act 1985 contained guidance to local authorities under section 604: Fitness for human habitation:

"In determining, for any of the purposes of this Act, whether premises are fit for human habitation, regard shall be had to their condition in respect of the following matters: repair, stability, freedom from damp, internal arrangement, natural lighting, ventilation, water supply, drainage and sanitary conveniences, (etc) and the premises shall be deemed to be unfit if, and only if, they are so far defective in one or more of those matters that they are not reasonably suitable for occupation in that condition."

• As with much other UK legislation, the Housing Act 1985 did not define "reasonably suitable".
• There have been many amendments to, and re-issues of, the Housing Act 1985. The most significant of these, for the purpose of this investigation, is the Local Government and Housing Act 1989.
• Schedule 9, clause 83, provides new wording for section 604 which includes (inter alia):

"... a dwelling-house is fit for human habitation for the purposes of this Act unless, in the opinion of the local housing authority, it fails to meet one or more of the requirements in paragraphs (a) to (i) below ... -
(d) it has adequate provision for lighting, heating and ventilation;"

• The 1989 Act did not define "adequate".
• The 1989 Act is available on the Office of Public Sector Information website, and Schedule 9 of the Act can be found directly from this link.

The current situation

• At the present moment we believe that it is theoretically possible for a new house to be built, complying with all relevant construction legislation, with windows less than 10% of the floor area. This house could then, on completion, be condemned by the local housing authority as "unfit for human habitation".
• It would appear that, for the last twenty years, it has been the responsibility of the local housing authorities (in England and wales) to determine what constitutes "adequate" provision for natural daylighting. Not surprisingly, many of them have opted for requirements based on the now-obsolete 1976 Building Regulations, hence the continuation of the "10%" rule-of-thumb.
• Also, not surprisingly, local housing authorities seem to have been working on the assumption that newly-built housing will have "adequate" natural daylighting and the "10% rule" occurs mainly in their requirements for the existing housing stock, particularly when setting standards for houses in multiple occupation.
• This was not an issue when the developers of new houses and flats wished to maximise the amount of window area and achieving the "10%" was easy.
• It has become a more serious issue, however, as the thermal requirements of the Building Regulations have encouraged smaller window sizes and more expensive glazing systems. Some speculative housing developers are now beginning to question whether they can reduce window sizes below 10% of the floor area - which will allow them to use cheaper glazing systems. Hence the original reason for this research.
• BS 8206:Part 2 (Lighting for buildings : Code of practice for daylighting) provides a great deal of general advice about windows including, in paragraph 4.4, guidance on the size and proportion of windows. It does not, however, include any reference to specific window sizes equivalent to the "10% rule".

The future

• The latest (2006) edition of ApprovedDocumentF changes many of the regulations relating to ventilation. These changes are driven primarily by fuel conservation issues and there are, as before, no references to natural daylighting.
• The Housing Act 2004 introduces a new approach to assessing 'hazards' in all domestic properties using risk assessment and new enforcement powers. It will repeal the fitness standards, in the Housing Acts since 1985, and all of the legislation relating to houses in multiple occupation. The list of 'hazards' includes "Lighting" but without differentiating between natural daylighting and artificial lighting.
• Once again the legislation is targeted at the existing housing stock, particularly rental accommodation and houses in multiple occupation, and appears to assume that newly-built residential property will be "adequate".
• Read about the Housing Act 2004 on the ODPM website from CHECK LINK.
• Read the Housing Act 2004 on the OPSI website from this link.

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