Converting Single Storey Extension to Double

In an effort to add value to your home, or to fit in that extra bedroom, study, or gym an affordable but effective solution is often extending the fabric of your current home from a Single-Storey to a Double-Storey. However, before you ready your wallet and your designer’s hat, here is an inside look into the essentials of this approach.

The First Step

An important first step is in understanding what you are working with. Adding an additional storey means an increase in the load upon your existing foundations so getting an expert to assess the foundations you are working can determine whether you are able to use the current foundations or whether they need to be deeper.

The process of examining your foundations is best in the hands of a Structural Engineer who will be able to assess their functionality in a way which escapes most Architects and Surveyors. Understanding the value of specialists is a key value at Extension Architecture, which is why we have an in-house Structural Engineer r eady to get things going!

At this stage, a trial pit will be dug to study your foundations. If your foundation to be adequate to support an added load, then you’re in for a smooth ride!However, if your foundations are not sufficient it is likely that you will need todiscard your foundation for a newer and shinier model. Interestingly, a common misconception is that this would be a more expensive approach, as in some cases this is a more affordable way.

Understanding Your Foundations

Now you know about how deep your foundations go—it’s time to bite your teeth into finer details so you are prepared to face the next steps!
Your foundation type is hugely dependent on the type of soil you have as some soils have lower bearing capacities than others. A foundation is placed to support the building’s movements in a way that will protect it and the structure. Below is a quick peak at three distinct foundation types.

AKA: ‘trench foundations’ or ‘slab foundations’


A simple and relatively shallow strip foundation works with more supportive soils and most modern foundations in London are concrete strip foundations. They are around 600mm wide and 1m deep.


  • Efficient and affordable
  • Low maintenance demands
  • Simple construction: excavated trench filled with cement to support walls


  • Prone to damage from tree roots
  • Can be expensive to repair
  • Offers minimal protection against bad weather

Raft Foundations

AKA: ‘waffle foundations’


These are more customisable foundations which are designed according to the soil. They consist of a reinforced concrete slab most commonly on a hardcore of a combined depth of 300mm.


  • Allows for a greater amount of weight on weak soil
  • Less depth required than slab foundations usually (economical)


  • Less common in the UK (could be trickier to construct!)
  • Prone to corrosion of steel reinforcement
  • Maintenance required

Pile foundations

AKA: Driven Piles, Displacement Piles


When deeper foundations are required, driven piles are often used to deal with moisture-heavy soils. Driven piles can be made of steel, pre-cast concrete, timber or combinations of these.


  • Easy fabrication
  • Easy installation
  • Immense structural strength


  • Can impact foundations in proximity
  • Noisier installation
  • May not be ideal for ground with poor drainage

Thinking Outside The Box

Recently, we were approached by a client to add an additional storey on top of the garage of a detached home.

He had been advised by two architects that his foundations were not deep enough but our planning expert offered the client an alternate approach, creating a fresh approach—maximising the profit by creating two semi-detached homes in the limited space which would be designed to optimise spaces.
This was done through a rigorous process marrying the practical side of planning with the ambition of designers, resulting in two modern homes with a height set to match the neighbours and accommodate space on all floors and unlocking the potential of the site.