Before the design stage can even begin it is vital to check if there is anything in, under or around the proposed area where you wish to extend that is likely to cause problems.

Marble Construction inspects the land for these problems when we initially come round to produce a quotation of how much your extension will cost. Many companies charge for this inspection, but it is free of charge from Marble and their experienced team is likely to spot problems other companies would miss. This makes the estimate Marble gives more accurate and allows the customer to have all the information needed to make an informed decision.

Below is a comprehensive list of possible problems with the site that Marble searches for when surveying for extensions.


Existing Foundations

The depth and quality of foundations can differ greatly. As a rule, Victorian houses have shallow foundations of under 450mm. Houses designs from post 1900 usually have a gradually increasing foundation depth.

This is important because the ground must be capable of supporting the additional weight you are adding on top of it without moving or sinking. This may mean that Foundations which are strong enough for a single story building cannot support a double story building. Similarly existing foundations may not meed today’s industry standards. This can increase the overall cost of extending a property.

To find out how deep the foundations are, dig a small hole next to an exterior wall of the property, be careful to avoid any pipes or cables that run underneath. This will allow expose what type of foundations were used and how deep they are.

Drainage positioning

For obvious reasons it is important not to damage any pipes that run under the property, to avoid this it is necessary to know the location of the piping and drains. This knowledge is also needed if you plan on adding a water supply or waste disposal to your extension (ie bathrooms, kitchens, toilets and swimming pools).

To find the location of Drainage pipes first try looking at the deeds to the property or asking the developer for the plans if it was a modern build. If none of these contain the necessary information, a drain survey might be the only option.

Start off by taking the covers off any manholes (inspection chambers) on the property. There should be one every time there is a bend in the pipe but this is not always the case.

Next, record the depths of the pipe from the cover to the bottom of the channel, the location and any branching pipes.

You can check what goes through each pipe by pouring different color food dyes into each room’s water removal, ie toilets, baths and kitchens.

The age of the property can also be a clue as to where the pipes may be. In Victorian properties shared sewers run straight through a row of back gardens. Each house’s drains run straight out of the property and will intersect this shared sewer at 90 degrees. In 1930s semi detached houses  the drain follows the side wall towards the street. Post war properties usually have toilets and kitchens at the front so waste pipes travel straight out of the front to the street.

If it is a rear extension to be a kitchen or to enlarge a kitchen, this can sometimes require a complete re-designing of waste water systems at the rear of the property due to complications with existing rainwater and kitchen drainage pipe locations.


Gas, Water and Electric supplies

Write down the location of your Gas, water and electricity meters and try to work out the route the cables and pipes would take in and out. For energy, Water and phone service, your suppliers should be able to provide a free site survey using their professional detection systems.

Cross analyse this survey with building plans for a higher level of accuracy. Regardless of the level of planning, it is very important to be cautious when digging or disturbing the ground in any way.


Wells and Cesspits

Despite being more common in pre 1900s housing, cesspits, wells and septic tanks are widespread amongst all types of property. If not found before construction, the results for any extension can be disastrous. First of all it is important to find out if they are in use, signs include if your water bill does not include a sewer charge and an even bigger clue is if it smells atrocious when the lid is lifted.

If they are in use, the cost of relocating them can be quite high as the existing cesspit will need to be de-commissioned as they may contain explosive methane gas.

Before construction starts, any wells, cesspits or tanks will need to be filled and capped with concrete. The foundations will need to contain a steel mesh to bridge the whole onto load bearing ground each side.

To prevent later subsidence, all ground in an 8000mm radius of the new build will need to be firm and free of cesspits, wells and soak aways.



Ordinance survey maps date back to the Victorian Era and can reveal buildings or features on the land you wish to build upon. The history of a site can reveal rare archeological finds or potential dangers such as possible pollution from heavy industries that used the land previously.


Quality of The Ground

They type of foundation required for a specific build varies based on how firm the subsoil is. South and South East England has a reputation for soft clay subsoil which can shrink in hot weather. This leads to foundation movement and cracking if they are too shallow.

To know for sure how stable the land is, dig a trial hole of between 1 and 3 metres in depth. This can reveal problems like previous back filling which may have taken place when the house was first constructed or previous to that. Signs of this include building rubble, pieces of china, bottles or tiles. Back-filled ground is usually softer than subsoil and in large amounts back filling can make an extension too expensive to be worth-while.



These can create problems on two fronts, the first being the roots causing the subsoil to contract and even breaking up the foundations, the second problem is that some species of tree are protected by law and cutting them down or even trimming them can lead to fines or worse. If the ground is home to protected plants or animals such as bats, newts or shrews it may be illegal to disturb their habitat.


Rights of Way

Obstructing footpaths or other rights of way can lead to very costly legal disputes and even being required to reverse any work in violation of public access routes. Check with the council before building across any land.



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