One of the first questions every homeowner asks when considering a renovation or extension is simple, how much will it cost to build? Unfortunately, the answer is rarely straightforward. Build costs vary significantly depending on the type of project, the size of the property, the level of specification, and the complexity of structural work involved.
To help at the early stages of planning, NU Projects built an online build cost calculator. It gives a realistic guide to construction costs, based on real project data.
A simple tool for real build costs
The NU Build Cost Calculator has been developed to give homeowners a clearer starting point before committing to design work or detailed cost planning. By selecting a few key variables, users can quickly generate an estimated cost range for their project.
The calculator considers three things:
- The type of project you are planning
- The size of the existing property
- The level of interior specification
From those three inputs it returns a construction cost range, drawn from residential projects NU Projects has delivered across London.
Built around real construction projects
The calculator covers the residential projects most often built in London:
- Kitchen extensions
- Double storey extensions
- Loft conversions
- Basements
- Side return extensions
- Wraparound extensions
- Full house renovations
Each project type carries different structural requirements, and the calculator adjusts the cost range accordingly. For example, basement construction typically involves complex structural works and waterproofing, while loft conversions require roof restructuring and structural reinforcement.
By structuring the calculator around real project types, homeowners gain a far more accurate understanding of potential costs than generic online estimates.
Understanding the specification levels
One of the biggest factors influencing construction cost is the level of interior finish. To help users understand this clearly, the calculator offers three specification levels.
Mid Range High quality engineered materials and reliable brands, such as Porcelanosa tiles and kitchens from suppliers like Martin Moore or Neptune.
High End Premium finishes including wide plank oak flooring, European marble surfaces, and luxury kitchen brands such as Bulthaup or SieMatic.
Super Prime Fully bespoke interiors with specialist joinery throughout and high performance appliances such as Gaggenau, Sub Zero, and integrated smart home systems like Lutron.
These tiers allow homeowners to see how material choices and design ambition influence the overall project cost.
Looking beyond the build cost
One of the most common misconceptions in residential construction is that the build cost represents the entire project budget. In reality, there are several additional costs that must be considered.
The calculator therefore also sets out the other parts of a project budget:
- Professional fees for architects, structural engineers, quantity surveyors and planning consultants
- Recommended contingency allowances
- Party wall surveyor costs where neighbouring properties are affected
- VAT applied to the project
By including these elements, the calculator provides a more realistic picture of the total project investment required.
Understanding the project timeline
Alongside cost estimates, the calculator also outlines a typical programme for delivering a residential project.
This includes the early design and survey stages, the planning application process, technical design and structural engineering, followed by the construction phase and final handover.
For many homeowners, this is the first time they see the full timeline involved in delivering a high quality renovation or extension, helping set realistic expectations from the start.
A better way to start a project
The NU Projects Build Cost Calculator is designed to provide clarity at the very beginning of a project. While it does not replace detailed cost planning or professional advice, it offers homeowners a reliable benchmark before engaging architects, designers, or contractors.
By understanding the likely cost and timeline early on, clients can approach the design stage with greater confidence and make better informed decisions about their project.
If you are considering a renovation, extension, or full home transformation, you can now explore the potential cost of your project in just a few minutes.
Try the NU build cost calculator and take the first step towards planning the project.
To talk to the team, send two lines through the contact form. You can also call 020 7731 6841. We usually reply within a few hours, weekends included.
Published 6 March 2026. Written by the NU Projects studio, edited since for house style and accuracy.