What has to happen before anyone picks up a shovel.

Before anything else, establish whether your project falls under permitted development or needs a full planning application. It changes the timescale far more than it changes the build.

• Permitted development is a set of national rules that let you build certain things without applying for planning permission — within limits on size, height, position and materials. Many single-storey rear extensions and a good number of loft conversions fall inside them.
• Those rights are narrower or removed altogether on flats, in conservation areas, on listed buildings, and where a previous owner has already used them up with an earlier extension. Some estates also carry an Article 4 direction or restrictive covenants that have nothing to do with planning law but bite just as hard.
• If you are relying on permitted development, it is usually worth applying to the council for a Lawful Development Certificate. It is not compulsory, but it is the piece of paper a buyer’s solicitor will ask for in ten years’ time.

A householder planning application has a statutory determination period, and a neighbour consultation period inside it. Neither can be compressed, so they belong at the start of your programme rather than the middle.

• You will need drawings — existing and proposed plans and elevations, plus a site and location plan. We can work with your architect or put you in touch with one, and we will tell you plainly whether a scheme is likely to be waved through or fought over.
• Councils consult neighbours and publish the application, so it is worth speaking to the people either side of you before they receive a letter about it. In our experience that single conversation prevents more objections than any amount of drawing.
• Build the determination period into your plans before you order anything with a lead time. A set of bi-folds arriving before you have consent is an expensive way to store doors.

Planning decides whether you may build it. Building control decides whether it is built properly. Nearly every extension, loft conversion and garage conversion needs building control approval even when planning permission is not required.

• Building control covers the things that keep the structure standing and the people inside it safe: foundations, structure, insulation and thermal performance, fire safety and escape, ventilation, drainage and electrical safety.
• Inspections happen at set stages — typically the excavation before concrete goes in, the foundations, the damp-proof course and oversite, the drainage, the structural work, insulation before it is covered, and a final inspection. A stage covered up before it is inspected can mean opening it back up.
• The completion certificate at the end is what proves the work was signed off. Keep it with your deeds; you will want it when you sell, and so will the buyer’s surveyor.

Anything that removes a wall, opens up a rear elevation or puts a bedroom in a roof needs a structural engineer’s calculations before it can be priced accurately, let alone built.

• The engineer sizes the beams and specifies the padstones and connections, works out what the foundations need to be, and — on a loft — designs the new floor structure to carry the loading a habitable room brings that a storage space never did.
• Those calculations are what building control assesses, and they also fix the cost of the structural stage. Pricing a knock-through without them is guesswork in both directions.
• It is worth getting the engineer involved early, because the answer sometimes changes the design. A slightly narrower opening, or a steel that sits above the ceiling line rather than below it, can make a noticeable difference to both the budget and the finished room.

If you are building on or against a shared boundary, cutting into a party wall, or excavating close to a neighbour’s foundations, the Party Wall etc. Act 1996 is likely to apply — and it carries notice periods.

• Notice has to be served on the affected neighbours in advance, and the required notice period depends on the type of work. Your neighbour can consent, or dissent and appoint a surveyor, in which case an award is drawn up before the work can start.
• None of this stops a project. What it does is add weeks if it is discovered late, which is the commonest reason we see an otherwise ready extension sitting at a standstill.
• A schedule of condition of the neighbouring property before you start is worth having even where notice is not strictly required. It is the only thing that settles an argument about a crack that may or may not have been there already.

Once approvals are in place, the build follows a fixed order. Most delays are not caused by the work itself but by something arriving at the wrong point in it.

• The sequence on a typical single-storey extension runs: set out and dig, foundations and inspection, drainage, oversite and blockwork to damp-proof course, structure and steels, roof, weathertight, glazing, first-fix services, plaster, second fix, decoration, then external works.
• Glazing is usually the long lead item — bi-folds, sliding doors and roof lanterns are typically made to measure, and they are measured from the built opening, not the drawing. That survey date sits on the critical path.
• Kitchens, staircases and stone worktops are the other three things worth pinning down early. All three are templated from the real building, and all three hold up the trades that follow them.