Over Garage House Extension - are they that easy?

Many estate type dwellings (detached & semi-detached) built within the last 40 years, often have a single storey side garage - sometimes attached, sometimes detached. This creates an opportunity for adding additional first floor bedroom or bathroom space over the garage.  There are many examples throughout the UK of this type of house extension.  So, are they the easy answer to adding additional floor space to a dwelling house?

Well, that can rely on a number of things.  Namely, is the existing garage structure (usually single skin brickwork) able to support the new first floor structure over (usually 300mm thick cavity walls plus roof and new floor loads).

In most instances, the garage was built using sub-standard foundations suitable for a two storey dwelling and the existing garage walls need so much remedial upgrade works that the over garage house extension becomes impossible or uneconomic without full replacement of the existing ground floor structure.

Therefore, a complete knock down & rebuild of the existing garage structure is usually required for over 50% of the these types of over garage house extension schemes.  It can often cost more money to underpin & structurally enhance the existing garage structure to be able to support the new first floor over than it is to remove it all and start a-fresh with a new two storey side extension for example.

Most homeowners who do not employ a proper house extension designer or architect right from the start are often not advised of this big issue and blindly go forward with an unworkable scheme through Planning that cannot be approved at Building Regulations.  This situation with over garage house extensions happens time and time again where the home owner has to resubmit amended plans for a complete new build two storey side extension rather than a first floor extension over the existing garage.

When many homeowners are faced with this probability, they then have to consider do they really need garage space?  If they are starting afresh, why would they spend thousands of pounds on a construction  simply to store a modern day car?

At last, the penny usually drops and a whole new area of home extension potential opens up to them.   Now the home owner can consider the formation of a separate reception room for say a study or open plan kitchen / diner.  The new two storey side extension now suddenly becomes a far more integrated option with better external design and internal use of space for all floors.

This is not to say that all over garage house extensions are difficult or impossible to construct as many are accomplished every year.  However, many end up looking exactly like the description of some compromised first floor element propped over existing garage with little scope for further improvement later on. 

These often cost just as much as a properly built two storey side extension simply due to the extent of structural upgrading required to the existing garage in order to support the new awkwardly positioned first floor element.

Our advise is for all homeowners considering an over garage house extension to also consider what could also be achieved with a full two storey side extension by removing the existing garage and analyse just how much additional useful habitable floor area you could obtain going down this route.