A ground floor, singe story extension may deliver what you need in terms of extra accommodation, but again, space comes into the equation. Can you afford to lose the amount of garden space your extension will require? If not, there is a third alternative which is to build a two storey extension.

A two storey extension may seem like a much larger project to undertake than a single storey one, but a two-storey extension is more compact in terms of footprint. It not only gives you more space, but it is more cost-effective than moving house and less expensive per square metre than a single storey extension. Two-storey homes are more cost-efficient to build because they offer more space without adding more foundation or roof costs, so the cost per square foot is lower. Similarly, a two storey extension will cost around 50%-75% more than the equivalent sized single storey extension.

Depending on the scale of your project and where you live in the country, the cost of a single storey extension will typically cost you somewhere in the region of £1,350+ VAT per square metre to build, excluding finishes, so a 3x5 metre single storey extension might well cost around £20,250+ VAT.

The current average cost of a double storey extension varies with the location across the UK, going from £2,500 to £3,500+ VAT per square metre, excluding finishes. In London the average cost excluding finishes is higher, with figures ranging from between £3,400 and £4,000+ VAT per square metre for the building cost.

When budgeting to build an extension, always remember that as well as the build costs, you will have professional fees such as architects for design and preparation of drawings, possible planning application, structural engineers, all of these before you build anything. Add to these the cost of finishes and decoration, flooring and fittings to give a total cost for the project.

While a two storey requires a higher overall cost, it offers a lower square metre cost than a single storey extension because the excavation and the roofing costs won't change from single to double storey extension. It offers the benefit of double the usable floor space of a single storey and as an added bonus, takes away less of your garden to deliver that space and as such is probably a better investment in terms of adding value to the property by offering extra accommodation without sacrificing garden space.

It is possible to build both single and double storey extension without applying for planning permission by using Permitted Development rights provided they meet certain criteria.

To build a two storey extension under permitted development rights you should first confirm with the local authority that you have permitted development rights, as these may have either been used previously or revoked by the council. Assuming you are able to uses these rights you will be required to work within the following;

• The area of extensions must be less than 50% of the total free area around the house
• The exterior design should blend in with the existing house and surrounding properties
• The width must be less than 50% of the width of the existing house
• The max length of rear extensions is 3 metres for terraced or semi-detached houses, or 4 metres for detached houses
• The height must not exceed the height of the existing house eaves.
• Only single-storey extensions are allowed, if the distance to the boundary is less than 2 metres
• Balconies and terraces are not permitted

If you are unable to meet all of these conditions you will be required to submit an application for planning permission. This can take upwards of eight weeks and at a cost of at least £250.

Regardless of whether you build under permitted development rights or apply for planning permission, your extension, single or double storey will require to comply with building regulations and on completion will need to be certified as meeting these regulations. Building regulations are designed to ensure that all building work is carried out in a fashion that will ensure the health and safety of anyone who uses the building in question. If you have the work carried out by a member of a competent person scheme, they will be able to self-certify compliance with building regulations.

As with all construction or installation related projects, always ensure that you check the credentials of the contractor you are about to use. Make sure they have the relevant experience, qualifications and insurances and check if they are members of a trade organisation or are on a Competent Persons Register.