Stamp duty, solicitors' and estate agents' fees, costs for a survey and now a Home Information Pack as well… An increasing number of homeowners are deciding that it makes more financial sense to enlarge the living space of their existing property than move house.

Is it worth it?
An attractive, well-built extension that adds square footage will also add value to your house. An extra room can put £30,000 onto a guide price, and the more sought after your postcode, the more value is added. But there is a limit: do not spend more than 10 per cent of the value of your property on an extension, or you may not recoup your investment.

What kind of extensions are the most popular?
Anything that creates genuine new space or reclaims inaccessible or uncongenial space: favourites are conservatories and lower ground floor extensions that conjure a bright kitchen-dining room from a dark basement.

Is a loft conversion for me?
These work well in urban terraces, where the homeowner wants more room without sacrificing any garden. Attics can be made light and habitable by installing rooflights, given extra headroom with dormers, or reclaimed as living space with the addition of a mansard – a flat-roofed extension upwards, usually built against the party wall.

How about a basement conversion?
Cellar conversions can cost three times the price of loft conversions, but they can also extend into the area underneath your garden, to add a space much larger than the footprint of the house. Basement conversions need planning permission, unless you are simply bringing an existing cellar into use, but consent is easier to obtain than permission to extend upwards and outwards, especially in a conservation area.

Talking of planning permission - do I always need it?
Not always, for minor extensions, but do check with your local authority before going ahead. If you live in a listed building or a conservation area, there may be restrictions on even minor work.

When can I get away without it?
It is always wise to check but, according to the government's planning portal, a ground floor extension that is no higher than four metres and not within two meters of a property's boundary does not require planning consent. In a terraced house, you will not need permission if your extension is less than 10 per cent the volume of your property; in a detached or semi-detached house, the figure is 15 per cent.

When must I have it?
You need planning permission if your addition will be higher than the highest part of the roof or will be nearer to any highway than the nearest part of the original house (unless there would still be 20 metres between your house and the road, even after the extension was built). If more than half the area of land around the original house would be covered by additions, once you had added your extension, you need to apply for consent.

Anything else?
In a terraced house, you may need permission from neighbours with party walls before work can start. And, even if you do not need planning permission, you still need building regulations approval for the work, to make sure your house has remained structurally sound. At the end of the project, you will need a Completion Certificate from your local authority – this is one of the documents that any future buyer of your house will ask to see.

Do I need an architect?
For everything but the simplest lean-to, it is advisable to retain an architect for a ground-level extension. If nothing else, a professional used to dealing with your local authority is more likely to get planning permission swiftly. On the other hand, there are some excellent specialist basement and loft conversion companies with in-house expertise, who can take you right from design to project completion

What will it cost?
Though you can add a small conservatory for under £5,000, for even a modest kitchen extension you should not expect much change out of £20,000. If you fancy a fabulous architect-designed glass garden box, say goodbye to the best part of £50,000. A simple basement conversion will set you back £20,000-£40,000, though good-sized London basements, with knocked through kitchen-living rooms and concertinaed glass doors onto the sunny new patio, regularly stray into six figures. By comparison, lofts are good value, starting at £17,500.

What are the pitfalls?
There are some additions that will not impress potential buyers: low-lit basement billiard rooms are not nearly as beloved as extra bathrooms or a handsome kitchen. If your local authority is on the ball, it will send an official to reassess your improved property and may move you up a council tax band.

What is the alternative?
If you find you cannot get planning permission or cannot afford the extension of your dreams, it may still be possible to grab some extra living space by knocking down an internal wall. Or, if it was an extra bathroom you needed, you could consider partitioning an existing bedroom to create an en suite.

Resources to help you plan
Planning regulations: www.planningportal.gov.uk
Online estimator: www.growyourhome.com
Royal Institution of Chartered Surveyors' free Party Walls Helpline, www.rics.org/partywalls
The Property Makeover Price Guide: A Guide to Organising and Budgeting for Home Improvement, various authors (RICS, 2006)
Home Extension Manual: The Step by Step Guide to Planning, Managing and Building a Project, by Ian Rock (JH Haynes & Co, 2007)

WORDS KATRINA BURROUGHS
APRIL 2008