Northampton Borough Council
Northampton Borough Council

Support to new external walls

2no 100 x 150mm deep concrete lintels (RMC) or 2no 100 x 140 pre-stressed lintels (Stressline) may be used to span the opening of a single garage door, the ends to be cut into the existing brickwork to give a bearing of 150mm each end.

The depth of any concrete foundation for the new front (or rear) walls will depend on the type and suitability of ground encountered but should not be less than 450mm below the finished ground level outside. It may need to be deeper if the driveway is to be taken up.


If the existing garage walls are only 105mm thick, the proposed wall
construction involving an inner leaf of blockwork or timber studwork
should incorporate a 50mm wide cavity, a reasonable thickness of
insulation with a vapour barrier on the warm side if timber studwork is used.

Where the garage is semi detached, sound insulation should
be provided to the party wall by the provision of a sound absorbent
quilt and two layers of plasterboard fixed to an independent stud wall with a minimum cavity of 10mm.

If a double garage is being partly converted, fire separation between the two parts should be maintained. The new separating wall should be suitably insulated.


The existing floor should be provided with a liquid or sheet membrane lapped into the damp-proof course of the wall construction and have a covering, incorporating insulation, ideally either a concrete screed (insulation to be 100mm polystyrene or 65mm Styrofoam) or a timber floor (floating, suspended or platform) (insulation to be 125mm polystyrene or 75mm Styrofoam)

If only a screed is provided over the insulation, the joints between insulation sheets should be taped and the screed should not be less than 65mm thick. The screed should also be reinforced with an anti-crack mesh.

Where a timber floor is to be used to overlay the existing slab and insulation is placed below the floor, a vapour barrier should be provided between the timber and the insulation.

A radon sump should be provided near to the new front wall which will connect the hardcore under the floor with a pipe (which can be capped off) to outside.

Roof / Ceiling

Traditional pitched roof - the roof should be insulated with not less than 100mm fibre insulation laid between the joists and 150mm laid across the joists. The roof should be ventilated at the eaves of the roof, equivalent to an area of not less than a continuous strip of 10mm wide, and at the top equivalent to an area of not less than a continuous strip of 5mm wide if the roof has a mono pitch design.
Flat roofs should be insulated with not less than 250mm of fibre insulation or 140mm of Styrofoam or 120mm of Celotex between the joists

The roof should be ventilated each side equivalent to an area of not less than a continuous strip of 25mm wide to give a complete cross flow of air with a minimum gap of 50mm over the insulation. If the flat roof be a warm roof construction, the insulation will be on top of the timber deck with the water / weather proof membrane above. In this case the insulation within the walls should be taken up to seal with the roof insulation (ventilation is not required in this design).

Window / door openings

A vertical DPC should be provided around the opening, sited between the front brickwork and the return of the blockwork. Preferably, this DPC should be fixed to the back of the frame. The DPC may also be part of the system used to reduce thermal bridging around the window opening i.e. bonded to expanded polystyrene (e.g. Damcor) or replaced with a plastic box section filled with insulation material (e.g. Thermabate).

If the new window is to be provided between original 225mm thick brickwork piers, the inside jambs of the window should be provided with wall board having an insulation material bonded to the rear of the board, fixed to the inside of the jamb with "dot & dab" fixing plaster. Similarly, the existing lintel above the existing garage door may be used for the new window should it be a type that is, or can be, insulated to avoid a cold bridging at the window head.

If the new room does not lead directly to the hall approaching the front door; the new window should be an "escape window" having an unobstructed opening of 0.33m², at least 450mm high and 450mm wide.

A wooden or UPVC framed window of glazed door should be a double glazed unit with a space between the glass of 16mm and a low emissivity coating, with draught-stripping around the opening elements.


Ventilation should be provided as follows:
Room Type Rapid Ventilation Background Ventilation Extract Ventilation
1/20th of floor area 1/20th of floor area 8000mm² -
Kitchen No minimum size 4000mm² 30 l/s adjacent to hob or 60 l/s elsewhere
Utility room No minimum size 4000mm² 30 l/s
Bathroom 4000mm² 15 l/s

The background ventilation may be provided by trickle ventilation within the window or by other means i.e. an air brick, but must be adjustable and located to avoid draughts (typically 1.75 above the floor level).


The new room should be fitted with a light fitting that will only accept energy efficient lamps.

Electrical installations

Any new electrical circuits should be installed by a registered self
certifying electrician or by a competent person who can issue an
electrical installation certificate under BS 7671

It is not possible to be precise in the advice we give regarding garage conversions as individual circumstances vary. Also, the above details provide only a selection of the more common alternative means of achieving compliance with the regulations and other solutions may be equally suitable. The Building Control Surveyor will be pleased to advise you on individual cases.

Building Control Contact details:

Telephone: 0300 330 7000
Fax: 01604 838 984
Address: Building Control, The Guildhall, St Giles Square, Northampton, NN1 1DE

Last reviewed : 04/05/2011

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