You’ve decided you want an extension. Great. Now the question everyone asks: how long is this actually going to take?

The honest answer is longer than most builders will tell you upfront. From the first conversation to the day you’re standing in your finished space, a single-storey extension typically takes 6 to 9 months. A double-storey build can push well beyond a year. That’s not scaremongering. That’s what the numbers show when you factor in every phase properly.

Key Topics Covered

  • Every phase from design to completion with realistic timeframes
  • Why planning permission takes longer than the legal deadline suggests
  • Party wall agreements and why they catch homeowners off guard
  • Construction timelines by extension type
  • The three biggest causes of delay and how to avoid them

Phase 1: Design and Drawings (2 to 4 Weeks)

Before anything gets submitted to the council, you need proper architectural drawings. This covers measured surveys of your property, planning drawings showing elevations, floor plans and site layout, and building regulations drawings if you’re running these in parallel.

For a straightforward single-storey extension, allow 2 to 4 weeks. More complex projects with awkward site conditions or bespoke design requirements will take longer. This phase costs money, but getting it right here saves weeks of back-and-forth with the council later.

Phase 2: Planning Permission (8 to 12 Weeks, Sometimes Longer)

Legally, the council has 8 weeks to decide a householder planning application. In reality, the system is under serious strain. According to 2024 data, local planning applications are averaging 19.2 weeks to process, well beyond the statutory target. Around 80% of councils are currently operating below full staffing levels, with a national shortfall of over 2,200 planners.

Plan for 8 to 10 weeks if your application is clean and straightforward. Plan for 10 to 14 weeks if there are any complications, neighbouring objections, or your local authority has a known backlog.

Some extensions fall under permitted development rights and skip the planning stage entirely. But always check first. Assuming permitted development applies when it doesn’t is an expensive mistake.

Phase 3: Building Regulations Approval (2 to 3 Weeks)

Planning permission and building regulations are two separate things. Planning covers what gets built. Building regs cover how it gets built.

Building regulations drawings are detailed technical documents covering structural calculations, drainage, insulation, and construction methods. Once submitted, a building control officer reviews and approves them. For a standard extension, allow 2 to 3 weeks.

The key point here: experienced builders run this phase in parallel with the planning wait rather than sequentially. That alone can save 4 to 6 weeks on your overall programme.

Phase 4: Party Wall Agreements (8 to 12 Weeks if Required)

If your extension is near a shared boundary or involves work to a party wall, you must serve formal notice on your neighbours under the Party Wall etc. Act 1996. The notice period for most extension work is 2 months before work begins.

If your neighbour consents in writing, you can proceed once the notice period expires. If they don’t respond within 14 days, or they dissent, a party wall surveyor must be appointed. From notice to signed party wall award, the process typically takes 8 to 12 weeks with cooperative parties, and longer if complications arise.

This phase catches a lot of homeowners off guard. Serve the notice early, ideally during the planning wait, not after you’ve got approval.

Phase 5: Construction (10 to 16 Weeks for Single Storey)

This is the phase everyone focuses on, but it’s the one that comes last. The build follows a clear sequence regardless of extension type: groundworks and foundations, walls up to damp-proof course level, drainage, superstructure and roof, first fix electrics and plumbing, plastering, second fix, then decoration and fitting out.

Typical timelines by extension type:

  • Single-storey rear or side extension: 10 to 16 weeks
  • Double-storey extension: 16 to 24 weeks
  • Wraparound or L-shaped extension: 20 to 28 weeks

Construction timelines extend significantly when bespoke materials are involved. Large glazed doors and roof lanterns routinely carry 10 to 12 week lead times. If you’re specifying made-to-order items, get them ordered before groundworks start, not after the roof goes on.

What’s the Realistic Total?

Adding every phase up honestly:

  • Single-storey extension: 6 to 9 months from initial design to completion
  • Double-storey extension: 10 to 14 months
  • Wraparound or L-shaped extension: 10 to 16 months

These figures assume no major delays. Weather, trade availability, unexpected groundwork issues such as buried drainage, and material lead times all have the potential to add weeks.

What Causes the Most Delays?

Planning delays are the most common culprit and largely outside your control. The best mitigation is submitting a thorough, accurate application the first time. Applications that go back and forth with the council for additional information can add 4 to 8 weeks on top of the standard determination period.

Changing your mind mid-build is the second biggest cause. Switching tile choices, altering window positions, or deciding to knock through an extra wall once the structure is up creates a cascade of scheduling problems across every trade on site. Finalise your specification before groundworks begin, not during them.

Not serving party wall notices early enough is the third. It’s a pure planning failure that adds weeks for no reason at all.

The Rosebrick Approach

We build realistic programmes from day one. That means accounting for planning timescales, running building regs drawings in parallel where possible, serving party wall notices early, and ordering lead-time items well before they’re needed on site.

The homeowners who end up frustrated are usually those who were given a 12-week timeline that ignored the 8 weeks of planning, the party wall process, and the building regs approval sitting ahead of it. We’d rather give you the honest picture at the start than explain delays later.

If you’re planning an extension in Mansfield, Nottinghamshire, South Yorkshire, or Derbyshire, contact Rosebrick Developments today. We’ll give you a programme that works in the real world.