Know exactly what you are applying for

Before you draw a single line, establish which approval route your extension actually needs. Many rear extensions fall under permitted development, but the rules are tighter than most people assume. A single-storey rear extension on a terraced house is limited to three metres beyond the original rear wall, four metres on a semi-detached or detached home, with a maximum eaves height of three metres. If you want to go up to six or eight metres, you will need prior approval under the neighbour consultation scheme, which is a separate process with its own forms and a 42-day determination period.

Permitted development rights can be removed entirely by an Article 4 direction, and they do not apply at all to flats, maisonettes, listed buildings, or homes within a conservation area where the extension is on a side or front elevation. In these cases you need full planning permission. If your proposal is borderline, or the site is sensitive, pay for pre-application advice from your local authority. A written officer opinion before you spend money on a full drawing package is usually the best value in the whole process.

The drawings your case officer will expect

Most validation delays come from incomplete or unclear drawings. Your architect or designer should produce a consistent set at a stated scale, normally 1:50 or 1:100, with a scale bar and north point on every sheet.

• Existing and proposed plans for every floor affected, showing the original house clearly and the proposed footprint shaded or hatched.
• Existing and proposed elevations of all sides touched by the works — not just the side you prefer looking at.
• At least one cross-section through the extension, showing the roof pitch, eaves and ridge heights, finished floor levels and the relationship to the boundary and any neighbouring windows.
• Dimensions on everything. Depth, width, height to eaves, overall ridge height and distance from the extension to the site boundary. Officers should not have to measure from the drawing.
• A materials key naming the brick, cladding, roof covering and window finish, or annotating the elevations directly.

On sloping sites, a section showing existing and finished ground levels is worth its weight in gold. It pre-empts the questions that otherwise arrive three weeks in about drainage, overlooking and retaining walls.

Site and location plans — the two documents people confuse

These are separate requirements and both are usually mandatory.

• Location plan: an Ordnance Survey-based extract, typically at 1:1250, with the application site outlined in red, any other land you own outlined in blue, and the surrounding roads named.
• Block plan: usually 1:200 or 1:500, showing the property outline, the proposed extension, boundary treatments, trees, parking, bin and cycle storage, and any drainage runs.

Do not crop a satellite screenshot or a Google Maps image — it will be rejected. Buy a licensed location plan from a mapping supplier or check your council's validation list for accepted sources. If the red line does not cover your entire ownership, you must show the rest in blue and complete the correct ownership certificate.

Statements, reports and certificates

A design and access statement is only compulsory for certain applications, but it is rarely wasted effort. Two well-written pages explaining why the extension is needed, how the scale and materials respond to the existing house, and what impact it has on neighbours, daylight and drainage can head off objections before they form.

Other documents may be required depending on your site:

• A heritage statement if the property is listed or within a conservation area.
• A flood risk assessment for sites in Flood Zone 2 or 3.
• An arboricultural statement where protected trees are nearby or on site.

On the forms, the most commonly missed item is the ownership certificate. You must serve notice on any owner who is not you, and the certificate must match the red line exactly. Get this wrong and the application is invalid, however good your drawings are.

Check the fee and the local validation list

Householder application fees in England have risen sharply in recent years, and Scotland, Wales and Northern Ireland set their own figures. Check the current amount on your council's website on the day you submit rather than trusting an old figure or a forum post. Paying the wrong fee is a guaranteed delay.

Every authority publishes a local validation checklist. Download it, work through it item by item, and tick each requirement off. It also tells you the accepted file formats and size limits, which matters more than you would think — oversized or low-resolution PDFs are frequently the reason an otherwise perfect submission bounces back.

Submit cleanly and keep talking to the officer

Name your files sensibly — "Proposed Elevations 01" is far easier for a case officer than "scan0004". Provide clear photographs of the existing house and its immediate surroundings, as these often answer questions that drawings cannot.

Once the application is validated, the clock starts. If the officer asks for an amendment, respond promptly and in writing, confirming exactly which drawing has changed and its revision number. Keep your neighbours informed as you go; a brief conversation over the fence has rescued more applications than any clever design statement. And when permission is granted, build to the approved plans — any material deviation needs a fresh approval, or you risk an enforcement notice years later when you come to sell.