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A Quick Guide to Extensions

Wall Footings

Footing construction for a cavity wall (Up to the DPC Level) should generally be :-

• A minimum 750mm deep but ideally 1000mm especially with shrinkable clay type soil.

• 600mm wide.

Any brick and block that will be below ground must be formed of either aggregate concrete block or bricks that are both Frost Resistant (F2) and have low Active Soluble Salts content (S2)  

Specific precautions should be made for any nearby trees, drains or poor ground conditions, depending upon location, and it is advised to seek professional advice regarding the treatment of these as additional agreements and permission may be required.

Cavity Wall

Typical cavity wall construction will be

·       100mm either medium density or aerated block,

·       75mm foil backed Celotex insulated board (taped at the joints and held tight against the inner block with retaining discs) ,

·       50mm air cavity and a

·       100mm external brickwork giving an overall width of the cavity wall of 325mm. This construction gives a U value of 0.18W/m2K.

(Ref. Kingspan web site and U value calculator)


A continuous Damp proof course (DPC) must be installed at a minimum of 150mm from ground level, and have a clear cavity extend below the DPC of 225mm

225mm stainless steel wall ties should be placed every 750mm horizontal and 450mm vertical centres, with additional ties within 300mm vertical centres at reveals.

Once fully constructed the internal surface of the cavity wall will either be plastered or have 12.5mm Plasterboard mechanically and Dot & Dab fixed.

Wall Openings

Where a door and/or window  opening is planned within the wall section a suitably sized and insulated lintels or beam must be placed over the opening with a cavity tray and weep holes immediately above the lintel on the exterior.

Depending on the size of the opening the lintel or beam must be set on a padstone and given the correct bearing length.

Cavity closers are required to be installed to close of the wall cavity around the reveals of each opening.


A typical solid floor construction will be constructed by first ensuring that there is a minimum of 350mm below DPC, with the final floor finish being level with the DPC

A typical solid floor will consist of:

·       150mm sub base typically MOT type 1 hardcore well compacted, with

·       a sand blinding covered with

·       1200 gauge damp proof membrane (DPM) linked with wall DPC.

·       100mm foil backed Celotex floor insulation, sealed at the joints with aluminium tape.

·       100mm Oversight or screed lightly reinforced with  – either a floating chipboard floor or floor levelling screed

Requirements to achieve a maximum not exceeding 0.18w/m2K vary depending on the ratio of external wall to floor area (P/A) and will need to be calculated as required.

Alternatively another option is to construct a beam and block flooring system where the beam and block is supported by the internal and external walls.

The Beam sections are inverted T-beams and manufactured to span specific distances. Beam depth and profiles are selected according to span and loading requirements, along with the type of block specified.

The Infill medium density or light weight concrete blocks are then positioned between the T-beams, and have a sand cement grout brushed over the surface, filling any gaps which is intended to seal the floor and avoiding movement in the blocks.


The table at the bottom (table 1) of this guide gives some guidance on suggested minimum timber sizes for floor joists, ceiling joists, rafters and flat roof joists.

If a truss type roof is to be used these should be designed, braced and fixed in accordance with BS 5268. The roof should be provided with horizontal lateral support and plate straps at 2.0m centres.

A traditional pitched roof with level ceiling should be insulated to achieve a U value not exceeding 0.15W/m2K. This can generally be achieved by providing a total of 250-300mm (depending on the manufacturer) quilt type insulation in two layers, the first laid between the ceiling joists and the second laid at right angles over the joists.

The roof should be ventilated at eaves equivalent to an area of not less than a continuous strip 10mm wide. If the roof is a mono-pitch also at high level with an equivalent area of not less than a continuous strip 5mm wide.

Breather membranes are becoming more popular as an alternative to traditional roofing felts but they must be installed correctly to perform at their best. This entails either draping the membrane between the rafters or using counter battens to help the membrane ‘breathe’ adequately and using a vapor check eg 500 gauge polythene before plaster-boarding the ceiling.

(Manufacturers advice should always be sought and followed)

A flat roof should be insulated to achieve a U value not exceeding 0.15W/m2K. Due to the thickness of insulation needed to achieve this, a warm roof, i.e. rigid board insulation placed on top of the joists is becoming more commonly used. The thickness of insulation needed will depend on the type you decide to use but could be up to 200mm.

As there are several ways of insulating a flat roof, manufacturers advice will often provide a practical solution. If a warm roof is not used, cross ventilation will be required. A plasterboard ceiling should generally be provided with a 500 gauge polythene vapour check.


Windows should be draught proofed and double glazed to achieve a U value not exceeding 1.4mw/m2K (e.g. Low E glass and Argon filled) or Window Energy Rating (WER) Band C. External doors should achieve a maximum U value of 1.4mw/m2K.

The door and window openings should be limited to 25% of the extension floor area unless the doors/windows achieve a better U value then the percentage can be adjusted accordingly. The area of any door/window covered by the extension can be taken off the area of doors/windows in the proposal to meet this target.

Glazing in doors and adjacent windows less that 1500mm above ground level and windows less than 800mm above ground level should have safety glazing.

For safety if there is not a clear path from the new room leading directly to the hall approaching the front door, the new window should be sized to allow escape. This will require an unobstructed area of 0.33m2 with a minimum dimension of 450mm high/wide and positioned between 800-1100mm above ground level to the bottom of the openable part of the window.

If the extension does not have its own external door, a fire detection and alarm system to BS 5839-6:2019 should be installed to the property.


Ventilation openings should be provided to each room. There is no minimum size for kitchens, utility rooms or bathrooms but other rooms require a ventilation opening of at least 1 /20th of their floor area.

Generally, background ventilation is required to habitable rooms at a rate of 8000mm2 and to kitchens, bathrooms, WCs and utility rooms at a rate of 4000mm2 .

Suitably sized mechanical ventilation ducted to external air or passive stack ventilation is required to bathrooms and shower rooms (15 litres per second), kitchens (60l/s or 30 if incorporated in a cooker hood), utility rooms (30l/s) and WC’s (6l/s).

Internal doors should be provided with a 10mm gap below the door to aid air circulation.


All new electrical work must comply with the requirements of BS7671 and a certificate issued that confirms the installation has been tested and meets the requirements of the building regulations Part P. The testing and certification must  be carried out by a person registered on the competent person scheme.


Soakaways to be installed a minimum 4500mm from any building and at least 2500mm from a boundary.


New rooms should be provided with a light fitting that will only accept energy efficient bulbs. Insulation between all building elements should be linked to prevent thermal bridging. Controls should be fitted to any new heating to prevent undue energy waste. Adequate sound resistance is required to certain stud partitions between a WC and living room or between a bathroom and bedroom and a minimum of 50mm insulation quilt between the studs and 15mm plasterboard would be sufficient.


These details are not intended to show you how to comply with the Building Regulations but are produced only as a guide to cover areas known to create problems on site, they show the more common means of achieving compliance with the Building Regulations but are not the only way to comply.

If your proposals vary in any way from these details, or you have any queries, you should discuss these with the area Building Control Officer before work commences.

We only recommend you submit your application using a Building Notice where you/ your builder are fully conversant with the requirements of the Building Regulations. If this is not the case, we recommend you employ a consultant to draw up full plans/ specification and submit a ‘Full Plans’ application on your behalf.


Table 1 Timber Sizes for rafters etc


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