Extending or renovating a home can be a daunting prospect. There are many factors to consider, from the requirements of the local authority, choosing a builder, the impact on you and your neighbours during construction, and not least the cost of the project.

Equipped with the right knowledge the building process can be a fascinating experience, culminating in the realisation of your ideas, and the personal and financial benefits of your new living space.

Some issues you may need to consider are:

Do I need planning permission?

Most house extensions will require the council's involvement in some way. Generally speaking, unless your plans fit the current permitted development criteria, you will need planning permission. Even if planning permission is not needed, you will probably still need to get the local authority to confirm that the work conforms to building regulations. Permitted development guidelines are published on your local authority's website and on the government website planningportal.gov.uk

Do I need building regulation approval?

Building regulation approval is entirely separate from planning permission. Building regulations are in place to ensure that building work, including plumbing and electrics, conforms to a minimum standard of workmanship and safety. Ultimately you will have problems selling your house if there are new additions or alterations that do not have building regulation approval. Your builder or project manager should ensure that the local authority is happy with all aspects of the building work as it progresses, and that site visits are conducted whenever necessary. The building inspector is acting in your best interests, but they also have the potential to delay your project if they feel something is not being done correctly.

Do different jobs have specific regulations?

Most new electrical work has to be carried out by a legally competent and qualified installer and signed off by either by them or the local authority. Any work on your gas supply or appliances such as boilers and cookers must be carried out by a Gas-Safe™ (formerly CORGI) registered tradesman. Replacement (but not renovated) windows will either need to be installed by a FENSA registered installer or notified to the council for inspection.

How much will it cost?

All projects are clearly going to have widely different costs, but as a rule of thumb, above-ground extensions (i.e. not basements) could be between £1,250 and £1,550 + VAT per m2 of floor space, however this depends on site conditions and the level of specification. A simple shell-only extension may be less than this, and if you are treating yourself to a gym, swimming pool and cinema room it might be upwards of £3,000 per m2! A typical dormer loft extension for a terraced house might be £25K-£30K, less for a rooflight conversion, and more for a mansard or hip to gable loft conversion.

Will an extension add value?

The majority of extensions will add more value to the property than the cost of the work: In S.W. London a property might be worth £4,000 to £10,000 per m2, but the cost of building additional living space will be far less. Basement extensions are significantly more expensive and generally only make financial sense in more expensive areas.

Will I need to budget for contingency?

Yes, normally allow between 5% and 15% for unforeseen expenses and changes to your specification.

Extend up, sideways or down?

Loft conversions are by far the most common extension in SW London. Not only are they frequently allowed under permitted development rules thus avoiding the planning permission route, but extending upwards on the existing building is usually the least expensive option. Side extensions need foundations and are therefore slightly more costly, and digging out the basement with the associated underpinning is the most expensive (perhaps three times the cost of an above-ground extension).

How long will the building work take?

Each project will have its own factors, but generally new build projects will be 12-18 months from design to completion, large extensions and renovations 6-12 months and smaller projects 2-6 months.

Will I have to move out?

The larger the scope of your building project, the larger the disruption to your normal life during the works. You may have to consider living without a kitchen, bathroom or even a toilet whilst your new services are connected. The heating may be temporarily affected, or electrical circuits interrupted. There will be dust, and dust is extremely difficult to contain. You will have to weigh the relative costs and benefits of moving out of a major project whilst the work is carried out. Moving out temporarily on a larger project will probably result in a faster and cheaper process as the builder will get access to the whole job site without having to move furniture and facilities around.

What about my neighbours?

It is a good idea to keep your neighbours informed of your plans. Neighbours will be consulted by the local authority on any planning application that might affect them, so it is in your interests to get them on your side early. You may find that you have to obtain their agreement to build close to their foundations, so remember to fill out any relevant paperwork (party wall awards, adjacent excavation notices etc.) in good time. Finally, make sure the builder is considerate to those surrounding the job site.

What specification should I choose?

This is the fun part. You might be surprised what you can achieve with even the smallest budget. Whatever specification you choose, allow a corresponding budget (plus contingency) at the outset. There is no point building your dream kitchen/diner and not being able to buy the cabinets.

Do I need a contract?

A contract is a binding document setting out the expectations of both parties in the construction process. Contracts set out the scope and specification of the work, the price and payment schedule, which party is responsible for planning matters, and responsibility for providing site facilities such as water, electricity and toilets. For medium to large projects, a contract should be in place before building work commences.

What if something goes wrong?

Nothing goes entirely according to plan. It is the job of the project manager to minimise the impact of the unexpected, and most of the time the project will ultimately continue with little inconvenience. For those rare times when something unforeseen severely impacts the project, site insurance, or contractor's all-risks cover is prudent.

Should I employ an architect?

Unless the work is of a purely structural nature, an architect can save you time and money as well as suggesting ideas you might not have thought of. If you don't feel you need the full services of an architect, you can employ a draftsman to draw up your planning drawings. A structural engineer will probably be required for most projects even if an architect is not.