Generally the area under the footprint of a new home should be graded and levelled to the required height specified under the house (this height would be the finished floor level of the house shown on drawings, less the thickness of the floor structure). This may require material from high areas under the house to be moved to areas which are too low. Excess ground and dirt usually has to be taken away, while a shortage of soil and gravel will require suitable material to be brought from elsewhere, usually at additional costs. Of course the material that’s used to fill and level the area under the building footprint must be suitable, and it must be compacted to the required engineer’s specifications. Clay, silt, topsoil, and material with vegetation or rubble, as well as material that’s inconsistent or which has large boulders, is generally unsuitable for fill under buildings. Excavated slopes and filled areas mustn’t be too steep so that they’ll collapse. These slopes usually require protection with vegetation or other methods so the ground isn’t eroded away.
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Before starting to level under the building footprint the area should be cleared of all topsoil, vegetation and unsuitable material.

It’s important to note that material that’s excavated from the undisturbed ground usually compacts to only about 80% of the original volume (this varies according to the type of ground and the desired amount of compaction). This means, that assuming the material excavated is all suitable for fill, then for every eight cubic metres (eight cubic yards) of fill required to level the platform for the house we have to excavate about ten cubic metres (ten cubic yards) from elsewhere. This is also a problem when trenches are excavated for pipes and cables. Often when the trenches are backfilled you may find there’s a shortage of material – nobody has stolen the material, it’s just been compacted down into a smaller volume and you’ll have to find additional material to make up the difference. 
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If additional fill material has to be brought from elsewhere (off the property) there’s an additional cost for the transport and often the material has to be purchased. It should be noted that a cubic metre (yard) of loose ground and gravel on a truck usually compacts down to around 65% (again this depends on the type of soil as well as the desired compaction). So for every six and a half cubic metres (six and half cubic yards) of fill required you usually have to purchase about ten cubic metres (ten cubic yards) of loose fill. (Always ask for expert advice, or test a few truck loads because with some gravel and earth you may be fortunate and the loose material compacts down to only 75 or even 80%.) When purchasing gravel and earth always check that you’re purchasing cubic metres (yards) not tons. If you’re paying for tons ask the supplier what that equates to per cubic metre or yard.

Unsuitable ground and material that’s in excess of requirements usually has to be carted from your property at an additional cost.

It should be noted that during the construction process earth fill material is usually required to backfill around foundations and under floor slabs. Therefore retain sufficient suitable material created while levelling for the house for later use during construction.

To limit the amount of material required to be bought for filling to level areas and in trenches, it’s important that all material excavated on the property, whether to install utilities or for foundations, is sorted, keeping good backfill material separate from unsuitable material. Unfortunately, often this isn’t monitored properly and the good material gets contaminated and becomes unsuitable for use.
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An engineer can advise on the compaction required under structures and the type and suitability of gravel and soil on the property.

Generally, where possible, it pays to design the house and set the house's floor levels such that the material excavated will all be used for filling, with minimal material to be carted off the property, or additional fill to be brought on.

If additional material is required, or you have surplus material, it pays to scout the area and nearby suburbs. Often there’re other construction projects nearby that have surplus material that they need to get rid of, or they may require additional fill material. Of course always check the quality of the material, since nobody wants a load of rubbish material that can’t be used and which then has to be removed at additional cost.

It should be noted that it’s illegal to dig holes on vacant properties or deposit excess material on them without the owner’s permission. But, sometimes even with the owner’s permission there may be permits to be gotten, such as environmental.

Where there’s some material that’s not very suitable, it may be a possibility to use this ground for filling lower down in trenches and under the house, and keep the better material for the uppermost layers – but obviously this is dependent on the engineer’s advice. 

On bigger properties, if there’s an excess of material it may be possible to incorporate some of this into the landscaping, creating low mounds.
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Of course below ground swimming pools and basements create additional ground which can either be used to level the property or it has to be carted away.

​​​This is an extract from the author's book 'An Introduction to Building and Renovating Houses: Volume 1 Hiring Contractors, Managing Construction and Finishing Your Home'.
​© 2020 This article is not to be reproduced for commercial purposes without written permission from the author.

​Paul Netscher has written 2 easy to read books 'An Introduction to Building and Renovating Houses - Volumes 1 and 2'. An Introduction to Building and Renovating Houses Volume 1 deals with Hiring Contractors, Managing Construction and Finishing Your Home.  and Designing your ideal home Volume 2 deals with Finding Your Ideal Property and Designing Your Dream Home. 
​("Great for those that DIY. Very helpful in home renovations!" said a Reader on Amazon.com 5*****)
These books are available from Amazon and other online bookstores in paper and ebook.

Steeply sloping ground can add to building costs since usually the area under the house has to be levelled, which could entail bringing in additional ground, or it could mean excavating into the slope to create a level platform. But creating a level area may also require retaining walls to be constructed to hold the banks or sides of excavations up. These walls can be costly and also unsightly. An alternative to reduce the amount of earthmoving is to design the house so that it has steps and that the various rooms are built at differing levels to accommodate and follow the sloping ground.
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Sometimes semi-basements can be constructed which are built into the slope to minimise the filling.

​Another solution is to build the house on stilts or columns so that the floor of the house, which is level, overhangs the sloping ground. The columns get longer further down the slope. This can be expensive, especially if the columns and house floor are constructed of concrete.

​But a steeply sloping property can create other problems which could include:

• Vehicle access to street level is difficult, and in particular vehicles with a low ground clearance could become stuck, or scrape on the driveway where it meets the road.
• Construction vehicles may have difficulty accessing the property, making construction more expensive if materials have to be moved from the road by hand to where they’re needed.
• Connections to the main sewer lines will be impossible if the existing town pipes are at a higher level than the lowest exit point from the planned house.
• Houses built below road level could be subject to flooding from stormwater from the road, or even from higher neighbouring properties.
• Maintaining a garden on a steeply sloping property is often more difficult than level properties.
• Many people don’t like purchasing steeply sloping properties, or houses with lots of steps, particularly if they are elderly or have mobility problems.

An advantage of building on steeply sloping ground is that if there’s a view there’s a possibility that the house in front of you will be lower and won’t obscure your view.