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Investigating subsidence problems with badly built conservatories


A conservatory is generally defined as having at least 75% of its roof and 50% of its walls comprising translucent material.
Conservatories are popular and there are many companies which specialise in their fabrication and installation.
They can be bought from a specialist company who will provide the complete design and construction service, including obtaining any necessary approvals. They can also be bought, off the shelf, from the larger DIY stores for the home improvement fanatics.  This does not usually involve the services of an Architect or a Structural Engineer but in some instances it perhaps should.

Most conservatories do not require planning consent or Building Regulations approvals and therefore they are often built off substandard foundations. It is the foundations which cause most of the problems after construction. The common belief is often (mistakenly) that it’s a lightweight structure so it won’t need deep foundations!

Other problems generally revolve around workmanship and materials, particularly the finishing off items which affect the final appearance.
The location of the conservatory can have an impact on the main building and put it in breach of the Building Regulations requirements (eg. fire safety access). These matters must be taken into consideration.

Conservatories are becoming more integral with the main house. When the original separating doors between the house and the conservatory are removed, it becomes a part of the house. and such a situation does require Building Regulations approval. This has a significant impact on compliance, especially in respect of energy conservation requirements.
Many of the comments and situations below apply equally to extensions to domestic buildings.

Planning Consent

Do I need planning consent for my conservatory?
There can be as many answers to this question as there are people whom you ask. The usual answer appears to be YES, BUT NO, BUT MAYBE.
YES - ordinarily you do require planning consent for a new building or extension.
BUT NO - because of Permitted Development Rights. This allows you to extend up to certain sizes and within certain constraints.
Up to 70 cubic metres or 15% of the original/1948 size - detached or semi-detached.
Up to 50 cubic metres or 10% of a terraced house.
Subject to a maximum in either case of 115 cubic metres.
Extension does not project beyond front wall of house.
Or is less than 20m from the highway.
Does not exceed highest point of roof.
Does not cover more than 50% of the total curtilage area.
BUT MAYBE - because there are circumstances where the Permitted Development Rights have been withdrawn, do not apply, or there are more stringent constraints.
Planning consent for the original building or later extensions may have withdrawn PDRs in order to limit overdevelopment of the site.
Listed Buildings will require consent for any changes or alterations.
In these areas there may be more stringent requirements on extensions.

  •    Conservation Area
  •    Area of Outstanding Natural Beauty
  •    National Park
  •    Norfolk Broads
  •    Suffolk Broads

Notwithstanding planning consent, there may be covenants in place which restrict development of your property.
The whole area is awash with varying advice and interpretation of what is allowed and what is restricted. If there is any doubt about planning consent it recommended that a Permitted Development Rights enquiry letter be sent to the local planning authorities. It is recommended that clients are asked to confirm that there are no restrictive covenants.

Building Regulations

Do I need Building Regulations for my conservatory?
Again the answer appears to be YES, BUT NO, BUT MAYBE.
YES - because its a building and should therefore be governed by the Building Regulations.
BUT NO - because there are exemptions for conservatories and small extensions :-
Statutory Instrument 2000 No. 2531
The Building Regulations 2000
Regulation 9
The extension of a building by the addition at ground level of -
(a) a conservatory, porch, covered yard or covered way; or
(b) a carport open on at least two sides;
in the case of a conservatory or porch which is wholly or partly glazed, the glazing satisfies the requirements of Part N of Schedule 1.
A conservatory is not defined but local authorities provide policy guidance on exemptions :-
It is a single storey ground floor extension
Has a floor area which does not exceed 30m2 3)
Has at least 75% of its roof and 50% of its exterior walls made of translucent material.
Has glazing that satisfies Part N of Schedule 1 of the Building Regulations 2000.
It is separated from the dwelling by existing doors or windows.
In the interest of energy conservation the conservatory should be unheated, but if fixed heating installations are proposed, they should have their own separate temperature and on/off controls.
BUT MAYBE - because there are a number of situations where the exemption does not apply because the safety of the main building would be jeopardised :-
a) If the conservatory is to be located in a position that will obstruct access by ladder to an escape window e.g. first floor inner room, second floor alternative escape window, etc. (Regulation 3 (2)material alteration affecting B 1- Means of escape).
b) If the conservatory is likely to obstruct access or facilities for the Fire Service (Regulation 3 (2)material alteration affecting B5 - Access and facilities for the fire service).
c) If the conservatory is likely to affect access or facilities for disabled people (Regulation 3 (2)material alteration affecting Part M - Access and facilities for disabled people).
d) If the position of the conservatory is likely to impair the air flow across a balanced flue and subsequently the correct functioning of a boiler (Regulation 3 (2) material alteration affecting a controlled fitting).


Conservatories are lightweight structures and therefore the foundation loads are generally low. This often leads clients and some builders to believe that shallow foundations will be satisfactory. This would be true in so far as settlement of the foundations is concerned, because adequate allowable ground bearing pressures for conservatory loads can usually be achieved with shallow foundations. But the problem is that shallow foundations are more susceptible to subsidence – the effects of seasonal variations in the ground and nearby trees acting on clay soils being the most prevalent causes of subsidence.
Typically most conservatories are built with foundations which are shallower than the Building Regulations would otherwise allow and are not inspected by a Building Control Officer or other knowledgeable person. Therefore it is not surprising that problems arise.
The Building Regulations say :-

Ground Movement
A2. The building shall be constructed so that ground movement caused by :-
(a) swelling, shrinkage or freezing of the subsoil; or
(b) land-slip or subsidence (other than subsidence arising from shrinkage), in so far as the risk can be reasonably foreseen, will not impair the stability of any part of the building

But this is further clarified by the Approved Document A :-
Minimum Depth of Strip Foundations
2E4 Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground. In clay soils subject to volume change on drying (‘shrinkable clays’, with Plasticity Index greater than or equal to 10%), strip foundations should be taken to a depth where anticipated ground movements will not impair the stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of foundations on clay soils should not be less than 0.75m although this depth will commonly need to be increased in order to transfer the loading onto satisfactory ground.

And the NHBC Standards give the best and most detailed advice with regard to foundation depths in shrinkable clays and where trees are close by. Soil samples have to be collected at various depths and tested to determine their Plasticity Index, this gives an indication of their Shrinkage Potential. Trees have to be identified and their distances from the foundations measured. The geographical location of the building has to be taken into account. These factors are fed into the equations, tables and graphs, and the result is a minimum foundation depth required to avoid subsidence. The minimum foundation depth may be as much as 3 metres or more.


The superstructure to most conservatories now comprises short cavity walls. with external brick to match that of the house, surmounted by UPVC framed, double glazed upper wall units and UPVC framed roof with double glazed units or cellular polycarbonate sheets. Sometimes the framing may be a hardwood. Cheaper variations omit the brick cavity wall and have framed panels for the full wall height.

The Building Regulations say :-
A1. (1) The building shall be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground -
(a) safely; and
(b) without causing such deflection or deformation of any part of the building, or such movement of the ground, as will impair the stability of any part of another building.
(2) In assessing whether a building complies with sub paragraph (1) regard shall be had to the imposed and wind loads to which it is likely to be subjected in the ordinary course of its use for the purpose for which it is intended.

In practise the superstructure rarely gives structural stability problems. If the conservatory is exempt, then it’s small, the roof spans are short and loads are low. If a larger conservatory, perhaps for a pub dining room or the like, then the Building Regulations will apply and the superstructure will be usually be designed by an Engineer and any standard conservatory components will be strengthened.
When there is foundation movement, the inherent flexibility of the superstructure (base walls excepted) allows considerable movement and distortion to occur without serious damage.
Problems with the superstructure generally come down to workmanship and finishing off.

Cavity trays

Where a conservatory roof butts up to an existing outside wall, a cavity tray should be installed to ensure there is no water penetration.
In practise a cavity tray is rarely, if ever, installed for a conservatory, and, perhaps surprisingly, this rarely gives a problem. This is because, for a cavity tray to be of use requires significant water to penetrate the outer leaf and run down the cavity. Such water penetration can only normally occur in walls exposed to high driving rain, and usually these have been given additional protection in their own right, such as tile hanging or render finishes.
What is seen with most of the ‘quality’ conservatories is lead flashings tucked into the bed joints where the conservatory roof abuts the house wall. This looks good and certainly adds to the ‘quality’ appearance, as well as stopping rain running down the external face of the wall entering into the conservatory.

Heat loss

Energy efficiency / conservation is the hot topic at present.
With a simple conservatory, attached to the house, but with the existing patio doors or whatever access, remaining in place there is no concern. But clients are wanting conservatories more integrated with the house, and are often using conservatories as a extensions in their own right – they appreciate the light and airy environment created this way.
Obviously the large glazed areas of conservatories are not good for retaining heat. Where the conservatory is to be integrated with the house, an assessment of the effect of the change has to be made. In most cases it is necessary to upgrade the insulation to other parts of the house to compensate for heat loss in the conservatory.

The Building Regulations require energy conservation measures and Approved Document L1 details three methods which can be used to show compliance. These are :-
The Elemental method where minimum U-values are specified for various elements (eg. pitched roof, walls, windows, etc.) and there are limits on the proportional area of door and windows to the size of the building. This can’t really be used when a conservatory is added because the area of glazing is too large.
The Target U-value method involves a survey of the property to determine the areas of the roof, walls, floors, doors and windows, and to identify their construction and any insulation to them. The heat loss of the various elements for the proposed arrangement is then calculated, added together and the Average U-value calculated. This is then compared to a the Target U-value which is calculated according to a formula in Approved Document L. If the Average U-value is less than the Target U-value, the insulation elsewhere in the building must be upgraded in order to raise the Average U-value. This usually means increasing the loft insulation, and sometimes insulating the walls.
The Carbon Index method is based on the Standard Assessment Procedure (SAP) rating. It calculates the annual carbon dioxide emission per unit area of the property.
All three methods include consideration of the efficiency of the heating source (boiler) and include variations which trade off more efficient boilers against lower insulation standards
Energy efficiency / conservation requirements are subject to review and standards are being raised at regular intervals. The next set of changes is expected in May 2006,and further changes can be expected beyond this.

Mike Royall is a Structural Engineer and can be contacted on:
Tel: 023 8072 0780

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