From idea to finished build: how a home project actually works

Most people start the same way: "I think I'd like an extension." Then comes a wall of unfamiliar words (architect, planning consultant, prior approval, party wall, building control, snagging) and no clear sense of what happens in what order, who you actually need to hire, or where the money goes.

This guide walks the whole journey, from the first vague idea to the day the work signs off and you release the final payment. It uses an extension as the running example because it is the most common project, but the same path applies to loft conversions, garage conversions and most home alterations.

Stage 1: "I think I want an extension", getting from a feeling to a brief

Before you speak to anyone, it helps to turn a vague want into a rough brief. You do not need detail, you need direction:

• What is the problem you are solving? "We need a bigger kitchen", "we want a downstairs bedroom", "the kids need their own rooms." The best designs solve a problem, not just add floor area.
• Roughly how much space, and where? Rear, side, two-storey, loft. You do not need measurements yet, just a sense.
• What is your honest budget? Including a contingency of at least 10 to 15%, because something unexpected almost always appears once walls are open.

Hold this loosely. A good designer will challenge it and often improve on it. The point of Stage 1 is simply to arrive at the next question with something concrete.

Stage 2: "What can I actually get?", the feasibility question

This is the question everyone wants answered first, and the honest answer is that it depends on three things, only one of which is general:

1. What the national rules allow. Some work is permitted development and needs no planning application at all; the rest needs permission. This part is the same for everyone and you can read the limits in our permitted development guide.
2. What your specific property allows. Conservation area status, an Article 4 direction, listed status, protected trees, flood zones or a planning condition on a previous permission can all remove rights or raise the bar. Two identical houses on the same street can have very different scope.
3. What your council actually approves. What is written in policy and what an officer signs off in practice are not always the same. The most reliable signal of what you can get is what has recently been approved, and refused, for projects like yours nearby.

A general guide can tell you the first of these. It cannot tell you the second or third, because those are specific to your address. That gap is exactly what a Planiverse property report is built to close: it checks the constraints that apply to your property and shows you what your council has actually decided on comparable projects nearby, so "what can I get?" stops being a guess before you spend money on drawings. Check your property →

Whether you do that check or not, you leave Stage 2 with a realistic sense of scope. Now you build a team.

Stage 3: Who you actually need, and the one distinction that confuses everyone

This is where most homeowners get tangled, so it is worth being precise. Not every project needs every professional, and the roles are not interchangeable.

The designer (you will almost certainly need one)

Someone to design the building and produce the drawings: for planning, for building regulations, for the builder to price from. There is a spectrum:

• Architectural designer: not a protected title, so quality varies. Can be excellent value for a straightforward extension. Check insurance and ask for similar local projects.
• Architectural technologist (CIAT): technically qualified, strong on the construction and building-regs detail, often more affordable than an architect.
• Architect (RIBA): fully qualified, regulated and insured, best for complex, sensitive or design-led projects.

For most extensions, a competent designer or technologist is enough. You step up to a chartered architect when the project is complex, the design ambition is high, or the property is sensitive.

The planning consultant (you probably do NOT need one, and here is when you do)

This is the distinction that trips people up. A planning consultant is not a designer. They do not draw your extension. They are specialists in the planning system: the policies, the arguments, the case for approval. Your designer normally handles the planning submission themselves for an ordinary householder application.

You bring in a planning consultant when the planning argument, not the design, is the hard part:

• The site is genuinely contentious or the proposal is borderline on policy.
• You are in a conservation area or affecting a listed building and a strong policy and heritage case is needed.
• You have already been refused and want to appeal, or to fight a policy point.
• The scheme is larger or more complex than standard householder work.

For a within-the-rules rear extension, paying for a planning consultant on top of your designer is usually money you do not need to spend. For a refusal you want to overturn, or a difficult heritage site, a good one can be the difference between approval and a dead end. Match the hire to the difficulty.

The supporting cast (project-dependent)

• Structural engineer: calculations for beams, steels and foundations, needed for almost any wall removal or extension.
• Party wall surveyor: only if the Party Wall Act applies (see Stage 9).
• Specialist consultants: heritage, arboriculture (trees), ecology, flood risk, only where your property's constraints trigger them.
• Builder / main contractor: builds it (Stage 10).
• Building Control: inspects the construction against building regulations (Stage 8); this is the council or a private approved inspector, not someone you "hire" in the usual sense.

Order of hiring: designer first (they shape everything), structural engineer when the design firms up, party wall surveyor and specialists as needed, builder once you have drawings to price from. Resist appointing a builder before you have drawings, because you cannot compare quotes for a thing that is not yet defined.

Stage 4: Design and drawings

Your designer produces a measured survey of what is there, then existing and proposed plans and elevations, a site location plan and a block plan, and, where required, a Design and Access Statement or (in a conservation area or for a listed building) a Heritage Statement.

Expect to iterate. The first design is a starting point, not the answer. This is the cheapest stage to change your mind in, so change it here rather than on site, where changes cost real money.

Stage 5: Pre-application advice (optional, often worth it)

Before submitting, you can pay the council for informal written feedback from a planning officer on whether your proposal is likely to be approved and what they would want changed. It is not binding and not a guarantee, but for anything contentious, or in a conservation area, it can steer you away from a refusal that would cost you far more in fees, time and redrawn plans. If the same officer later determines your application, they will already have engaged with it.

Stage 6: Submitting the planning application

If your project needs permission (your Stage 2 work told you this), your designer usually handles the submission, normally through the Planning Portal. You will need the application form, the drawings, site and block plans, any required statements, and the fee.

The council first validates the application (checks everything is present), then passes it to a planning officer. The clock that matters is the statutory target: 8 weeks for a householder application.

Stage 7: The wait, and the decision

During the determination period the council notifies immediate neighbours (who have 21 days to comment), publishes the application, and the officer assesses it against policy and any comments. Most householder applications are decided by the officer under delegated authority, not by a committee.

The officer may come back asking for amendments. That is usually a good sign. It means they are looking for a way to approve rather than refuse, and your designer should handle that dialogue. One polite follow-up after a few weeks is fine; constant chasing is not.

Three outcomes:

• Approved with conditions: the most common result. Read the conditions carefully. Some are pre-commencement conditions that must be formally discharged before any work starts, including demolition or site clearance. Starting work before discharging one can invalidate the whole permission. This catches people out regularly.
• Refused. You can revise and resubmit (often free within 12 months for a similar scheme) or appeal to the Planning Inspectorate. Appeals take many months and only around a third of householder appeals succeed, so resubmission is usually the better first move unless the refusal is clearly unreasonable.
• Withdrawn: sometimes the sensible choice if the officer signals refusal and you would rather revise.

Time limit: permission normally lasts 3 years from approval. Make a material start within that window or it expires.

Stage 8: Building Regulations (separate, and almost always needed)

This is the step people forget. Planning permission controls whether you build and how it looks. Building Regulations control how it is constructed: structure, fire safety, insulation, drainage, ventilation, electrics. They are an entirely separate approval with their own fee, and almost all structural work needs them even when planning permission does not.

Two routes: a Full Plans application (your design is approved before work starts, giving certainty, recommended) or a Building Notice (you start and are inspected as you go, quicker to begin but riskier). Inspection is by the council's Building Control or a private approved inspector; both are valid. Key inspections happen at foundations, damp-proofing, drainage, steelwork, insulation and completion. At the end you receive a Completion Certificate, which you must keep, because your solicitor will demand it when you sell.

Stage 9: Party wall matters

If your work involves cutting into or altering a shared wall, building on the boundary line, or excavating near a neighbour's building, the Party Wall etc. Act 1996 applies, and you must serve formal notice. The notice periods are precise and frequently misquoted:

• Two months' notice for works to an existing party structure (cutting into, raising or rebuilding a shared wall).
• One month's notice for building a new wall on the boundary line, or for excavations near a neighbour's foundations.
• Your neighbour then has 14 days to respond. Silence counts as dissent, not consent.

If they consent in writing, no surveyor is needed (do a photographic Schedule of Condition anyway, to protect both sides). If they dissent or stay silent, surveyors must be appointed: either one "agreed surveyor" for both of you (cheaper) or one each. You, the building owner, pay all reasonable surveyor fees, including your neighbour's. That surprises people.

The single best move here is to talk to your neighbours before serving formal notices. A friendly conversation with the drawings in hand dramatically raises the chance of simple written consent, which removes the surveyor cost entirely. And serve early: leaving it until the builder is due to start either delays the build or breaks the law.

Stage 10: Quotes and appointing a builder

With permission, building regs (or at least full plans submitted) and party wall matters in hand, you can price the work properly.

• Get at least three written quotes, each working from the same drawings and specification so they are comparable.
• A good quote is itemised, not one lump sum, so you can see foundations, structure, roof, electrics, plumbing, plastering and finishes separately.
• Ask for references from recent similar projects and actually contact them.
• Check public liability insurance (and employer's liability if they have staff).

Red flags: a large upfront payment before any work, no written quote, no references, no insurance, or a quote that dramatically undercuts the others (usually hidden extras later).

The contract: never start without a written contract. For householder work the JCT Minor Building Contract or the FMB contract are widely used. It should reference the drawings, state the total price and what is included, tie a payment schedule to completed stages (not dates), set start and completion dates, say how variations are priced, and include a defects liability period and a retention clause.

Retention means holding back around 2.5 to 5% of the contract value for 6 to 12 months after completion, so the builder has an incentive to return and fix snagging. Stage payments mean you never pay more than the value of work actually completed on site. Both protect you, and both are worth insisting on even if a builder resists.

Stage 11: During the build

• Inspections get booked at each key stage. Make sure your builder is booking Building Control inspections proactively, because work that is covered up before inspection may have to be exposed again, which is expensive.
• Watch for CIL. Some councils charge a Community Infrastructure Levy on new floor area over 100 square metres. There is a self-build exemption for extensions to your own home, but you must apply for it before starting work or you lose it.
• Keep records. Photograph everything that will be hidden: foundations, steel, insulation, membranes, drainage. Useful for Building Control, insurance and any future dispute.
• Get variations in writing before the work is done. Mid-build changes are normal, but verbal "while you're here" extras are the single most common source of disputes between homeowners and builders.

Stage 12: Completion, sign-off and snagging

The home straight, and the stage where money discipline matters most.

• Final Building Control inspection. When the builder considers the work complete, Building Control does a final inspection and, if it passes, issues the Completion Certificate. Keep it with your other property documents. It is a legal record you will need when selling.
• Snagging. Before you release the final payment, walk every room and list everything not finished properly: cracked plaster, paint touch-ups, doors that do not close, unfinished sealant, scratched glass. Give the builder a written snagging list and a reasonable deadline (usually two to four weeks). The retention is released only once snagging is resolved.
• Update your records. Tell your home insurer (your property has changed size and value) and your mortgage provider if your terms require it. File everything together: planning permission, Building Regulations completion certificate, drawings, party wall agreement, the builder's contract, and any guarantees or warranties.
• Warranties. Expect a workmanship warranty from the builder (typically one to two years) and manufacturer warranties on products (windows, boiler, roofing). For structural work, consider an insurance-backed guarantee that protects you if the builder ceases trading.

That is the full arc: a vague want at one end, a signed-off, documented, insured finished build at the other.

The mistakes that catch most people out

1. Starting work before discharging a pre-commencement condition. This can invalidate the entire permission. Read your decision notice.
2. Forgetting Building Regulations. Separate from planning, and needed even when planning permission is not. Skipping it creates insurance and resale problems.
3. Serving party wall notices too late. Two months for party-structure works. If the builder is ready and notices are not served, you are stuck.
4. Paying the builder ahead of the work. Never pay more than the value completed on site.
5. No written contract. Verbal deals are unenforceable and breed disputes.
6. Hiring a planning consultant you do not need, or skipping one you do. Match the hire to the difficulty of the planning case, not to nerves.
7. Letting the 3-year permission lapse. Make a material start in time or reapply, possibly under changed policy.
8. Losing the Completion Certificate. Replacing it later is slow and awkward, and your buyer's solicitor will insist on it.

Frequently asked questions

In short

A home project runs idea, feasibility, team, design, permission, building regs, party wall, build, sign-off. None of it is mysterious once it is laid out in order; the trouble comes from doing things in the wrong sequence, hiring the wrong professional for the difficulty of the job, or skipping a step (building regs, condition discharge, party wall notice) that quietly turns into an expensive problem later.

The one step a general guide cannot do for you is the feasibility question at the very start: what your property can actually get, given its constraints and what your council has approved nearby. Settling that before you commission drawings is what turns the whole process from a gamble into a plan.

Related guides

• Do I need planning permission? — when permission is required, and when it isn't.
• Permitted development rights — what you can build without a full planning application.
• Article 4 directions — where your council has removed permitted development rights.
• Planning permission costs — application, professional and building costs, with current figures.
• Building regulations — how a project is signed off as safe, and the certificate you'll need when you sell.
• Working with a builder — choosing, managing and protecting yourself on a build.