House Extensions

Often not, for a single storey rear extension on a house. Rules called permitted development let you extend without applying, within limits. The main ones:

• A single storey rear extension can go up to 4 metres beyond the original back wall of a detached house, or 3 metres on any other house, and be no more than 4 metres high.
• Close to a boundary, the height at the eaves is capped lower, and nothing can be higher than the existing roof.
• Extensions and other buildings together cannot cover more than half the land around the original house.
• A side extension must be single storey and no wider than half the width of the original house.
• Materials must look similar to the house. Verandas, balconies and raised platforms are not covered.

The original house means as first built, so an earlier owner's extension uses up part of the allowance.

Bigger rear extensions: prior approval

For a single storey rear extension you can go to double those depths through prior approval. You notify the council, it consults the adjoining neighbours, and it decides whether the effect on them is acceptable. Do not start until you have that decision in writing. The route is not open in a conservation area or a National Park.

Conservation areas, Article 4, flats and listed buildings

In a conservation area, every side extension needs planning permission, the prior approval route is closed and cladding the outside is not permitted development. An Article 4 direction can remove rights on particular streets, and some estates lost them by a planning condition when they were built. Flats and maisonettes have no permitted development rights for extensions. A listed building needs listed building consent for an extension, whatever its size.

If you build under permitted development, consider a lawful development certificate. It is optional, but it is what a buyer's solicitor asks for when you sell. The government's technical guidance has a diagram for each rule, and our guide to planning, building regulations and listed building consent explains how the three differ.

The Party Wall etc. Act covers three things an extension often involves: building on or astride the boundary line, cutting into a wall you share with next door (to seat a steel, say), and digging foundations close to a neighbour's building and deeper than theirs. Each needs written notice to the neighbour before that work starts. The Act sets minimum notice periods, so serve it as soon as the drawings are settled.

The notice comes from you as the owner. Your neighbour can agree in writing, or dissent, and then a party wall surveyor, or one for each of you, draws up an award saying how the work is done. Talk to the neighbours before the letter arrives: it makes the notice a formality. We build to whatever the award says, and the start date on our quote follows it.

Setting out and foundations. The footprint is marked out from the drawings and the trenches dug. The building control inspector looks at the open trench and agrees the depth before any concrete goes in. On firm ground that is simple. On soft ground, clay or near trees it can go deeper than drawn, which is why the foundation is priced as a provisional sum.

Drains and the floor. Any drain under the footprint is dealt with now, as the drawings show. The walls come up to damp proof course, then the floor goes in: hardcore, a membrane, insulation and a concrete slab, or a beam and block floor, with the screed later.

Walls. Cavity walls go up with insulation in the cavity, an inner leaf of block and an outer leaf of facing brick to match the house, or block to be rendered. Find the matching brick before anything else is ordered. Cavity trays and weep vents go over the openings, and the new walls are tied into the old ones. More on brickwork.

Steel and lintels. Lintels go over the new windows and doors. Where the back wall of the house opens up, the wall above is propped, the opening cut, and the steel set on padstones to the engineer's design. The rooms behind are sheeted off first.

We do not publish a figure for extensions, per square metre or any other way. What moves the price:

• the ground, and how deep the foundations end up
• how wide the opening into the house is, and the steel it needs
• the roof: flat or pitched, and how many roof lights
• the glazing: large sliding or folding doors against a standard door and window
• brick to match, or block and render
• drains under the footprint that have to be moved or protected
• access: on a terrace with no side way, everything comes through the house
• where our price stops: at plaster, or with the floor and the kitchen too

Anything that cannot be measured until the ground is open, the foundation above all, belongs on the quote as a provisional sum. That is an allowance, adjusted up or down to what is really needed, with the basis written down. Ask every builder you are comparing:

• Which items are provisional sums, and how will they be adjusted?
• Are scaffold, skips and taking the spoil away included?
• Who arranges building control, and who pays its fees?
• Are the steel, windows, doors and roof lights supplied, or only fitted?
• Does the price stop at plaster, or include the floor, decorating and kitchen?
• What happens if the engineer changes the steel or the inspector deepens the footings?

The way we put a quote together is on how we price a job.

BCP Council is the planning and building control authority for Bournemouth, Christchurch and Poole, and runs both services. Wimborne, Ferndown, Verwood and Corfe Mullen come under Dorset Council. New Milton, Lymington and Ringwood are New Forest District Council, and inside the National Park the National Park Authority decides. National Park land counts like a conservation area for permitted development, so side extensions there need permission and the prior approval route is closed.

The ground changes from street to street. Much of Bournemouth sits on sand, where trench sides can slump, so the trenches are dug, inspected and concreted with as little gap as possible. Parts of Poole and Christchurch are on clay that shrinks in a dry summer and swells in a wet winter, and there the inspector may ask for deeper footings, as the guidance on foundations explains.

Trees matter on both. Roots draw water out of clay, so a big tree near the footprint, yours or next door's, can mean much deeper foundations. Mention any at the first visit. BCP's own appraisal of the Meyrick Park and Talbot Woods conservation area names pine trees and tree-lined avenues as a prominent part of its character. In a conservation area you must give the council notice before cutting a tree back, and a tree under a preservation order needs consent. Damaging a protected tree's roots to fit a footing can be an offence, so the foundation design has to allow for it.