A well-thought out garage conversion can add as much as 10 per cent to the value of your home, and is one of the most cost-effective ways to improve your property’s resale value. An additional benefit is increased living space without incurring the costs and inconveniences of moving house. Below, you’ll find the information needed to plan and carry out your garage conversion.

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Design and Space Planning

The internal space of most garages is longer and thinner than most people prefer their rooms. To achieve a more natural shape for the conversion, consider using stud or block walling to convert the garage into two rooms. The additional room is often used as a toilet or shower. A storeroom suits others better. Consider the use you plan to put the rooms to and either make some drawings yourself or get some made.

Planning Permission

Planning permission is unnecessary if you don’t plan to alter the structure of the building. If you live in a Listed building or a Conservation Area, planning permission may be required for even minor modifications. Homes in newly built estates sometimes require planning permission.

Standalone garages are more likely to require ‘change of use’ planning permission when converted to habitable rooms. Check with your local planning department by phone. If the answer is favourable, write the next day and ask for written confirmation.

Building Regulations

The change of use from a garage to a habitable room will mean compliance with building regulations, including delivery of a building notice to your council. Building regulations apply to ventilation, moisture proofing, insulation, fireproofing, escape routes, and structural soundness. As a result, almost any design decision must take them into account.

When dividing up the garage a new room is created. This room is subject to a set of building regulations that require an escape route and ventilation separate from the main room. Alterations such as an infill wall replacing the original garage door will also be subject to building regulations concerning the foundations. The building inspector will want to visually inspect windows, doors, fireproofing and foundations before he gives a certificate of completion.

Insulation and Damp Proofing

Insulation is assessed in terms of a U value: the measure of the rate of heat escape, in watts, from a square metre of the material in an hour. Walls must achieve a u value of 0.35W/m²/K, and roofs a u value of 0.16W/m²/K if pitched or 0.25 W/m²/K if flat and the floor 0.25W/m²K. Windows should be 0.18 W/m²K. The floor will need to be 0.25W/m²K.

Window manufacturers will give the u value as part of the specification of the windows.

Damp proofing will be necessary in the walls and floor. In the wall, a damp proof membrane needs to be laid between two layers of bricks to prevent moisture rising up the wall by capillary attraction and rotting the brickwork. The membrane is usually polyethylene or a bitumen polymer. In many garages a damp proof course will have been laid in the walls already, but check this. The floor will need to be damp proofed by laying a damp proof membrane at the same time the floor is remade.

Plumbing and Wiring

Make a thorough survey of the plumbing and wiring in the house and garage. Any wall you plan to pierce for doorways or windows needs special attention. Locate the main outflows for water, and, if you plan to install a toilet, the soil outflow.

Check the garage for wiring in the walls and ceiling. Rewiring the garage for lights and electric radiators will place additional strain on the household mains, which is fused at 100 amps. An additional mains supply can be installed but the cost is highly variable, running between £500 and £20k. This will also require the installation of a separate consumer unit.

Otherwise, locate the garage on the current consumer unit. If it doesn’t have its own MCB (miniature circuit breaker) consider replacing the consumer unit or upgrading it. If the garage is to be another habitable room in your house (the most common use for a garage conversion) its own MCB is probably enough. Consider adding at least one new 20-amp circuit. Wiring to a detached garage can be run through an underground conduit. If it is to be a separate dwelling, a new connection may be required, depending on likely power usage; consult an electrician.

Flooring

The existing garage floor will be sufficiently strong but will require additional damp proofing and insulation to meet building regulations.

A concrete floor can be created and a damp proof membrane added between the two layers. Insulation is also added under the new floor, but sometimes requires a separation layer to prevent a chemical reaction with the damp proofing. Finish concrete floors with a screed of 75mm depth and seal before painting.

A raised timber floor can be built over the existing floor. This will require a gap of 150mm between the joists and the original floor. Wide or long raised timber floors may also require a small supporting wall. Damp proofing is laid under the timber and insulation between the joists and the timbers of the floor. Fire regulations may require a step in a raised timber floor at doorways, to prevent fire spreading under fire doors by burning of the floor timbers.

Walls

External walls are covered by building regulations and must meet requirements in terms of moisture-proofing and insulation. If the garage is integral to the house the exterior walls will usually meet building regulations. Otherwise consider a second block wall, or a stud wall, inside the existing exterior wall. Insulation and power and water lines can be put behind this wall. Note that this will decrease the interior dimensions of the space.

Interior walls between rooms in the conversion must meet building regulations’ requirements for fireproofing. This can mean one or two layers of fireproof plasterboard on stud walls. For block walls this is unnecessary. Doors through interior walls need to be fireproof, with a 30-minute rating. Additionally, building regulations may require a step in the floor at the doorway to prevent fire spreading along the floorboards.

If the garage will see a lot of use, especially if it is external to the house, consider additional insulation. This will decrease cost long term. Additionally, the insulation requirements for buildings have been rising and will continue to rise; over-insulating now will make your property more saleable later when building regulations may require it, and it’s cheaper to add while the walls are under construction.

Replacing the garage door with an infill wall will require investigating the foundations to confirm their depth. Shallow fill foundations may require improvement to support the additional weight. Consider replacing the door with a large window and panelling. The adjoining room to the door can be a store room, or the window can be the ‘front’ of the conversion. By saving the expense of reworking the foundations these options can be considerably cheaper. Some owners report having built an interior-style block wall behind the existing garage door and insulating this, leaving the exterior appearance of the garage unaltered.

Windows and Doors

A second room will require ventilation to meet building regulations and an escape route to meet fire regulations. Building regulations require a window 1/20 the floor area of the room, while fire regulations require a window with a 600mm base opening and a total area of not less than 0.45m². The window must also have trickle vents.

Windows that meet these requirements are available in metal frame or uPVC, though metal frame windows must have a ‘thermal break’ (because of metal’s conductivity) to meet insulation requirements. Wood frame windows can meet requirements too, but they have to be of sufficient depth to accommodate a 24mm double-glazed unit. Frequently a wood effect uPVC is used, or metal frame with block and wood cladding.

When installing windows, get a contractor accredited by BSI, CERTASS or FENSA (FENestration Self Assessment). Make sure the contractor shows you the certification.

Doors must meet the same requirements as windows in terms of their u value. In addition to uPVC doors with double glazed windows, wooden doors are also a good choice.

When putting in doors or windows in the walls of the garage, reinforcement may be necessary to the original garage wall. This will be in the form of a concrete lintel or a rolled steel joist (RSJ) depending on the size of the new opening. A lintel will need to overhang the edges of the window gap by 100–150mm.

In standalone garages particularly, footings are often absent or inadequate and this makes adding a window and some panelling a more attractive idea than digging new footings and building an infill wall. However, the front opening of most garages is not a standard size for windows so additional building work may be necessary. A block or brick wall filling some of the gap can bring the opening to a standard size without requiring additional footings, due to the decreased weight when compared to a full infill wall. Consult the building inspector at the planning stage.

Completion Certificate

Any project that requires a building notice will require a completion certificate. The completion certificate is simply proof that when the work was completed, it met regulations. However, having a completion certificate does make a difference to the value added to your property by the garage conversion. Estate agents and prospective purchasers will value your completion certificate highly since things like an under-floor damp proof course can’t be proven without one.

When the building inspector makes his final inspection, you must submit to him the documentation proving that building regulations were met in the construction and installation of the parts he has not visually inspected. The corgi certificate for the boiler, the NIC or EIC for electrical installations and the paperwork for the insulation, damp proofing and the FENSA or other certification for the contractor who installed the windows will all need to be shown. Once the building inspector is satisfied the completion certificate should follow, technically within 28 days but often much sooner.

Garage Conversion Pros

Design Control: Converting a garage means every step of the design process is under your control, subject to technical and legal restrictions.

Added Value: Moving house costs money that can’t be recouped. But converting a garage into a habitable room adds more value to your home than it costs in most cases.

Local Amenities: Moving children from school and families from commute routes, sports and leisure facilities and places of worship is difficult and sometimes expensive. Remaining at your current address while acquiring additional living space is a better solution for many.

Contract Control: When you carry out the work and who does it are your decisions. 1 in 10 house sales falls through because one party changes their mind. If you choose conversion you’re the sole decision maker.

Council Tax: Moving from a 3 to a 4 bedroom house could put you up a council tax band. A garage conversion leaves council tax bands unaffected.

Garage Conversion Cons

Disruption: During garage conversion one or more existing rooms will frequently be rendered unuseable by building work. Rooms adjoining the garage, and garden or yard space, will be most affected. Workmen will be present in your home during working hours. In addition to direct work on the conversion, electricians and plumbers will require access to the mains supplies inside and outside the house.

Control of Work: The householder will be held responsible for the legality of work done on his property. In supervising work, being present to allow workmen access and making design and other decisions, time and energy will be required.

Impact on Existing Buildings: Extending the garage will consume garden space. Light to existing rooms, access to the property, yard space and the exterior appearance of the building as a whole may all be affected.

Planning Uncertainty: Projects which require planning permission may not receive it. Application will involve a non-returnable fee, usually of about £150. This will be higher if you want to alter a listed building or you live in a conservation area.

Cost Uncertainty: Once investigations begin on your property, you might be required to pay for additional improvements or repairs. Damage resulting from building work might require repair; delays and unpredictable setbacks might elevate costs far beyond budget. However, conversion is likely to remain the cheaper option: moving house usually incurs costs of about £10-15k (considerably more in London) in addition to the cost of the new house itself.

By contrast, conversion leaves the householder in profit in property value terms.

This article is sponsored by Ben Holland and Stephen Green, directors of Holland And Green Architectural Design – A company on a mission to Transform People’s Lives by Creating Wonderful Living Spaces. Follow Holland and Green on twitter @HollandandGreen and on Youtube.

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