A well-thought out garage conversion can add as much as 10% to the value of your home. Expect to pay between £5,000 and £8,000 for converting the average single garage, making it 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:

Design and Space Planning

At five metres long by two and a half metres wide, the internal space of most garages is longer and thinner than most rooms in a house. To achieve a more natural shape, consider using stud or block walling to convert the garage into two rooms, typically a toilet, shower or storeroom.

Consider also how you plan to use the rooms, 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, so a garage conversion is permitted in most circumstances. However:

  • If you live in a Listed building or a Conservation Area, planning permission may be required for even minor modifications.
  • Some new build homes were built with a condition requiring the garage to remain as parking, so an application to remove it becomes necessary.
  • Standalone garages are more likely to require ‘change of use’ planning permission when converted to habitable rooms.

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What insurance do I need if I am converting a garage?

If you are carrying out garage conversion works and are managing the project yourself you should arrange conversion insurance to cover the new works and the existing structure. This is because most home insurers will exclude loss or damage whilst the property is undergoing alteration or renovation.

Site insurance caters for both the existing element of the property that’s being converted and all the new conversion works that go into the process. The existing structure is usually your house – so if the is damaged during the works the site insurance will cover it and completely replaces the requirement for buildings insurance, which is not suitable.

All the works, including any temporary works, materials, plant tools and equipment need to be covered. Public liability and employers liability is automatically included to ensure you are adequately protected.

Conversion insurance needs to be in place from the moment you plan to start works on the property and should continue to the point the project is completed and taken into full use.

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:

  • moisture proofing
  • ventilation
  • insulation
  • fireproofing
  • escape routes
  • structural soundness.

As a result, almost any design decision must take them into account. For example:

  • When you divide 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 or she gives a certificate of completion.

Once the building inspector is satisfied, the completion certificate should follow within 28 days. It is often much sooner.

Insulation and Damp-Proofing

Insulation:

  • In pitched roofs, go for two layers of 150mm glass fibre quilt, one between the joists, another over as usual.
  • Flat roofs tend to need one layer between of rigid PUR insulation board and another below — the space in between flat roof joists however can’t be entirely filled. A 50mm air gap must be left above for ventilation. The second layer underneath will drop the ceiling height a bit. Typically 150mm deep flat roof joists will receive 100mm of PUR insulation between the joists and 50mm beneath them.

As with floors, if the ceiling height is an issue (due to smaller than 150mm deep joists allowing less insulation between) multi-foil laminate insulants can help reduce the thickness of the under layer.

Damp-proofing: The concrete floor may or may not have been cast over a damp-proof membrane (DPM). In recent decades, integral garages would normally have included a DPM and certainly the walls would have a damp-proof course (DPC). But without plaster and screed finishings to conceal them, the two elements would not meet as they would in the house. Protecting the concrete floor with a polythene or paint-on DPM and dressing it up under your new finishings to the DPC layer will ensure that damp is not a problem.

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, with the cost varying from £500 to £20,000. 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 miniature circuit breaker (MCB), consider replacing the consumer unit or upgrading it. If the garage is to be another habitable room in your house, 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

Floor insulation is always absent in a garage and including some in the conversion should be part of the project, whenever it’s possible. That said, breaking up the concrete floor simply to re-cast it over is usually uneconomic and unnecessary.

Garage floors are often lower than the house floor and a layer of insulation beneath a floating chipboard deck floor can be selected to bring the two levels together, particularly when the difference is up to 120mm or so. Chipboard sheets are at least a third of the depth of a floor screed, giving you depth for more insulation. Rolled out multi-foil laminate insulation is thin and good for draping over fixed-down battens when the difference in levels is not so great.

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 costs 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 much cheaper. Some owners have built an interior-style block wall behind the existing garage door and insulated 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.

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.

Cost Effective Way to Add Space: Conversion costs for the average garage are between £5,000 and £8,000.

Local Amenities: Moving children from school and families from local facilities and communities is difficult and sometimes expensive. Remaining at your current address but with additional living space is a better solution for many.

Contract Control: One 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 three to a four 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.

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

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 that 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.

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