The Pre-Build Process Explained: What to Get Right Before Your Builder Starts

There's a truth that experienced builders know and most homeowners find out the hard way: the most expensive mistakes in any construction project happen before the first brick is laid. A poorly considered brief, drawings that don't match the planning rules, structural calculations done after the quote — each of these can add weeks of delay and tens of thousands in unexpected costs.

At Fortis Construction, we've seen the difference a well-managed pre-build process makes. Projects that arrive at the construction phase with everything in order run smoother, finish closer to programme, and cost closer to the original budget. This guide walks you through every stage of the pre-build process — so you go into your build with confidence.

Step 1: Define Your Brief and Set a Realistic Budget

Before you approach an architect or builder, you need a clear idea of what you want. Not just "a bigger kitchen" — but the specific rooms, sizes, layouts, finishes, and function you're aiming for. The more specific your brief, the more accurate your initial cost estimates will be.

At the same time, set a budget range — not a precise figure, but a realistic ceiling. Be honest about what you can comfortably afford, including a contingency of at least 15–20% of the build cost for unexpected findings (which are almost inevitable once walls come down or ground is broken).

A good rule of thumb for Hampshire and Surrey in 2025: budget at least £1,800–£3,000 per square metre for a quality single-storey extension, and £2,500–£4,000+ per square metre for more complex or premium builds.

Step 2: Appoint an Architect or Design Consultancy

Unless you're doing a very simple project within permitted development limits, you'll need professional design drawings. These serve three purposes: they turn your ideas into something buildable, they form the basis of your planning application, and they give builders an accurate scope to quote against.

At Fortis, we work alongside Planning By Design — a multi-award-winning planning and design consultancy operating across the UK and Ireland — as part of our pre-build partner network. They specialise in architectural design, planning applications, permitted development assessments, building regulations, and structural engineering calculations, and they help our clients get the design and planning right before we put tools to ground.

What to expect from this stage:

• Feasibility study: What's actually possible on your site, within planning rules and budget?
• Concept design: Initial layouts and options for you to choose between
• Planning drawings: Detailed plans, elevations, and sections required for a planning submission
• Building regulations drawings: More detailed technical drawings required for building control

Architectural design fees typically range from £1,500–£6,000 for extensions, and £5,000–£20,000+ for new builds, depending on complexity.

Step 3: Understand Your Planning Position

Before submitting anything, your design consultant should confirm your planning position: does your project require full planning permission, or does it fall within permitted development (PD) rights?

Most single-storey rear extensions on detached homes fall within PD rights (up to 4 metres beyond the original rear wall). Semi-detached and terraced houses are limited to 3 metres. Side extensions are limited to half the width of the original house and must be single storey under PD.

Even if your project falls within PD, we strongly recommend applying for a Lawful Development Certificate (LDC) — a formal confirmation from your council that the works were lawful. This costs around £103 and protects you when you sell.

If full planning permission is required, budget for an 8-week determination period and a possible round of revisions if the council requires changes. Planning applications in Hampshire and Surrey are handled by district-level authorities, each with their own character policies and design guidance — local knowledge matters enormously here.

Step 4: Structural Engineering Calculations

Any extension involving the removal of a load-bearing wall, a new opening, or a structural frame will require a structural engineer's calculations. These specify the size, type, and support requirements for any steelwork, and are required by building control before certain elements of the build can proceed.

Structural engineering fees for a typical extension are usually £500–£1,500. For more complex projects — underpinning, new structural frames, or multi-storey builds — costs can reach £3,000–£6,000.

Don't skip this stage or try to cut costs here. Incorrect structural specifications are a significant safety risk and can invalidate your building regulations sign-off.

Step 5: Submit for Building Regulations Approval

All extensions and new builds require building regulations approval, regardless of whether planning permission was needed. Building regulations cover:

• Structural integrity and loading
• Fire safety and means of escape
• Thermal insulation and energy efficiency
• Drainage and water disposal
• Electrical installation standards
• Ventilation
• Accessibility

You can submit a Building Notice (simpler, work starts relatively quickly, but inspections are more frequent) or a Full Plans application (detailed plans submitted and approved in advance — the safer route for larger projects). Your builder or design consultant will advise which route is appropriate.

Step 6: Consider Party Wall Obligations

If your build affects a shared boundary, shared wall, or ground excavation within 3–6 metres of a neighbour's property, the Party Wall Act 1996 may apply. You must serve a Party Wall Notice on all affected neighbours before work begins — typically 2 months before the start date.

If a neighbour consents, work can proceed. If they dissent or don't respond, a Party Wall Award must be agreed through appointed surveyors (one per side, or a single agreed surveyor). This process typically costs £800–£2,000 and adds 6–8 weeks to your pre-build timeline. Build it in.

Step 7: Site Surveys and Ground Investigations

For most extensions, your builder will assess ground conditions during the initial site visit. For new builds, larger extensions, or sites in areas with poor soil, made ground, or trees nearby, a formal ground investigation may be required. This typically involves trial pits or borehole investigations and costs £500–£3,000.

The results directly inform your foundation specification — and getting foundations wrong is one of the most expensive mistakes in construction. A modest investment in ground investigation at this stage is always worthwhile on larger projects.

Step 8: Choose and Appoint Your Builder

Only once your planning, structural engineering, and building regulations drawings are in order should you approach builders for detailed quotes. This is a critical point — inviting builders to quote without drawings produces wildly variable, largely meaningless estimates. With drawings in hand, you can get three detailed, comparable quotes and make an informed decision.

What to look for in a builder:

• Evidence of similar projects completed successfully
• References you can actually call — and a recent project you can visit
• A detailed, itemised quote — not a lump sum
• Public liability insurance (minimum £2 million)
• A proposed programme with start and completion dates
• A clear payment schedule tied to milestones

How Fortis Supports the Pre-Build Process

At Fortis, we don't just show up when the groundworks start. We're involved from the early stages — helping clients understand what's feasible, referring them to the right design and planning consultants within our trusted partner network, and ensuring every pre-build stage is completed before our team mobilises.

The result is builds that start right and finish as planned. If you're at the early ideas stage and you're not sure what's possible, get in touch and we'll talk through your project — including whether a referral to our pre-build consultancy partners would be useful. It's a conversation worth having early.