Top Project Cost Estimation Tools for Home Extensions & Renovations
If you're planning a home extension, renovation, conversion or commercial fit-out, one of the biggest early questions is: “How much is this going to cost me?”
It’s a fair question—and a crucial one. But it’s also not as simple as plugging a few numbers into a calculator.
At DSN+BD Construction, we’ve seen time and again how underestimating costs—especially by focusing only on the "build price"—can lead to stress, delays, and unwanted compromises. That’s why we encourage all clients to look at the entire scope of their project and understand what the real numbers are likely to be.
Here’s how you can start getting to grips with your total project cost—and some tools to help.
Start With a Feasibility Study – Our First Recommendation
At DSN+BD, we always recommend beginning with a feasibility study carried out by your project architect. This is one of the most valuable—and often overlooked—steps in the process.
A feasibility study lays the groundwork for everything that follows. It’s where we assess what’s possible, what’s practical, and what it might cost.
✅ What’s Included in a Feasibility Study?
A measured survey of your existing property
Initial concept sketches or layout options
High-level discussions around materials, finishes, and systems
Review of planning policy, site constraints, and development potential
Early-stage budget guidance and cost bands
Professional advice tailored to your goals and property type
🎯 Benefits of a Feasibility Study
Clarity from the outset – you’ll understand your options before committing to a full design
Avoid wasted time or cost – we quickly flag what’s unviable, risky, or overly ambitious
Make better decisions – informed by planning context, structural input, and realistic costs
Control your budget – with early cost insights to guide your scope from the very start
Confidence to move forward – knowing you have a solid foundation for planning and design
Think of a feasibility study as a “pre-project roadmap” that gives you clear sight of the road ahead—before you step on the gas.
What Should Be Included in a Full Project Cost Estimate?
Before we look at tools, let’s break down what’s typically included in the true cost of a building project. Spoiler: it’s a lot more than just bricks and labour.
✅ Architectural Design + Planning Fees
Initial design and feasibility studies
Planning applications or permitted development documentation
Measured surveys
Structural engineer input
Building regulations approval
✅ Construction Costs
Groundworks and foundations
Structural work
Internal fit-out (walls, ceilings, floors)
Kitchens, bathrooms, joinery
Mechanical & electrical services (plumbing, heating, electrics)
Decoration and finishes
✅ Fixtures, Fittings & Equipment (FF&E)
Appliances, sanitaryware, lighting, tiles, flooring materials
Door hardware, ironmongery, and other fittings
✅ External Works
Landscaping, paving, fences
Driveways, drainage
Access improvements
✅ Professional & Statutory Costs
Party wall surveyors
Building control fees
CDM / health & safety compliance
✅ VAT (Usually 20%)
Don’t forget: most residential building work is subject to VAT at 20%—a huge consideration if you're not factoring it in.
✅ Contingency
We always recommend budgeting 10–15% contingency for unforeseen issues or scope changes during the build.
Tools & Resources to Estimate Project Costs
1. Build Cost Calculators
These online tools give ballpark figures based on project type, size, and location. They’re useful for early-stage planning but should always be taken with a pinch of salt.
“Build It” Cost Calculator
Great for self-builds, extensions and renovations. Offers good detail around build quality assumptions.Love Renovate Extension Cost Calculator
A user-friendly tool for estimating home extension costs in the UK, with adjustable settings based on region, size, and specification.“Homebuilding & Renovating” Cost Calculator
Ideal for estimating costs on a per-square-metre basis.
Pro Tip: Be ware that some tools may leave out VAT, design fees, or contingency—so use them as a starting point, not the whole picture.
2. RICS Cost Estimators & QS Advice
If you want a professional pre-construction independent budget plan, consider hiring a Quantity Surveyor (QS) or using RICS cost estimating tools.
These services provide detailed breakdowns, especially helpful for larger or more complex projects.
You’ll receive line-by-line costs and insight into where money is being spent.
At DSN+BD, we often collaborate with independent QS partners when clients want deeper pre-construction budget control.
3. Architect-Led Budgeting
Architects—especially those working in design-led design and build environments like ours—can give early design-stage estimates informed by:
Recent completed projects
Cost per square metre based on design intent and complexity
Buildability insights that avoid budget blowout later on
Working with a team like DSN+BD Construction, where design and construction are integrated from day one, means your budget is considered as carefully as your layout.
Why DSN+BD Helps You Budget Better
We believe you deserve clarity from day one—not surprises halfway through.
That’s why we:
Start with a feasibility study to explore options and shape realistic expectations
Include the full scope of works in our project costings
Break down budgets clearly, including VAT, design fees, and contingency
Use real data and experience to guide realistic allowances for finishes and fittings
Don’t omit items just to make our quotes look cheaper
We’d rather give you the truth early than the panic later.
Want Help Creating a Realistic Budget?
Whether you’re planning a £75k extension or a £300k full-house renovation, we can help you understand:
What it will really cost
What to allow for
Where to invest
Where to save
And how to get the most value from your budget
At DSN+BD Construction, we don’t just build spaces—we help you build confidence in every decision you make.
Let’s talk about your project, your priorities, and how to bring it all together—starting with a smart, tailored feasibility study.