How accurate is the ES Architecture Cost Calculator?

The calculator provides an indicative construction and architectural fee estimate based on current market rates and 250+ completed projects across London and Hertfordshire. It offers transparent benchmark figures for Base, Premium, and Luxury specifications to help you plan your budget before committing.

How much do architectural services cost for a house extension in London or Hertfordshire?

Architectural fees typically range from 5% to 12% of overall construction value. At ES Architecture, we break this down into clear fixed-fee stages: Phase 0 (Feasibility), Phase 1 (Concept Design & Planning), Phase 2 (Building Regulations & Technical Drawings), Phase 3 (Tender & Contractor Procurement), and Phase 4 (Site Oversight).

What is the average cost per square metre for an extension in London and Herts?

In 2025/2026, standard single-storey extensions range between £1,800 and £2,400 per m² (Base Finish). Premium architectural finishes with bifold doors, underfloor heating, and bespoke glazing range from £2,400 to £3,200 per m². Luxury specifications with structural glass, complex steels, and high-end joinery exceed £3,200 per m².

Does the calculator estimate include planning and building control fees?

The calculator covers architectural design fees and physical construction estimates. Statutory council fees—such as the standard England householder planning application fee (approx. £258) and Building Control inspection fees—are separate statutory costs which we advise on during Phase 0.

Can ES Architecture handle both architectural design and physical construction?

Yes. Through our turnkey construction arm, Prilux Construction, we provide a complete end-to-end design and build solution. This ensures seamless handover from drawings to site, guarantees fixed-budget accountability, and prevents costly contractor disputes.

ES Architecture • Prilux V10 Engine

Project Cost Calculator & Services Estimator

Get an immediate, transparent investment estimate for your house extension or renovation. Calculate physical construction costs alongside RIBA Chartered architectural services and statutory council approvals.

Select Your Finish Specification

Defines the material specification, glazing, and trade craftsmanship on site.

★ We recommend the Premium Finish for all family homes

Rental / Entry Specification

Base Finish

£2,350 / m² new build

Standard building-grade PIR, uPVC double glazing, trade laminate, and basic Building Regs compliance.

★ Prilux Benchmark (Middle Service)

Premium Finish

£2,950 / m² new build

Slimline aluminium bi-folds, engineered oak, bespoke quartz kitchen, and designer wet-room sanitaryware.

Architectural Masterpiece

Luxury Finish

£3,950 / m² new build

Triple-glazed structural frameless glass, oversized pivot doors, artisan microcement, chef's kitchen & smart AV.

1. Project Scope & Sizing Dimensions

🏡 New Build Extension Footprint:

m²

The newly added ground or first-floor extension footprint.

0 m² £2,950 / m² (Premium Finish) 150 m²

🖈 Existing House Refurbishment Area:

m²

Internal areas remodeled (re-plastering, rewiring, steels knock-throughs, flooring).

0 m² £1,480 / m² (Premium Finish) 200 m²

Combined Total Internal Floor Area: 60 m² (646 sq ft)

⚖ Project Site & Planning Complexity:

Affects structural engineering, planning rigor, and architectural technical detail.

Low Complexity

Single-storey rear, level garden, standard Permitted Development.

0.90x Factor

Moderate (Default)

Detached/semi-detached, structural steels, open-plan knock-through.

1.00x Baseline

High Complexity

Conservation Area, Listed, Green Belt, Thames Water, tree roots.

1.25x Factor

2. Key Indicative Fixtures Joinery & Plumbing

🍽 Kitchen Supply & Installation:

🛁 Bathroom / En-suite Count:

Total Combined Project Investment

£188,775

Gross Investment (inc. 20% VAT): £226,530

On-Site Physical Construction: £177,950

ESA Professional Services & Statutory: £10,825

On-Site Physical Build Breakdown

New Extension Footprint (35 m²): £103,250

Existing Internal Refurbishment (25 m²): £37,000

Kitchen Supply & Installation: £14,500

Bathroom Suites & Wet Room Tiling: £6,800

Physical Build Subtotal (ex VAT): £161,550

Pre-Construction & Approvals

ESA Professional Services & Fees

Calculated on project size and site complexity, starting at £1,800 per phase for an RIBA Chartered Practice.

Phase 1: Survey & Planning Application £2,080

Measured building survey, Leica laser scan, 3D spatial concept, planning drawings & council application submission.

Phase 2: Building Regs Technical Pack £2,040

1:50 construction layouts, 1:20 cross-sections, insulation/drainage notes & statutory Principal Designer appointment (Building Safety Act).

Phase 3: Tender & Contract Administration £5,200

Builder tender vetting, JCT contract preparation, valuation certificates, and weekly site inspection visits.

⚙

Structural Engineer Calculations £1,650

Structural steel beam sizing, load-bearing knock-throughs, and foundation depths for Building Control sign-off.

⚖

Council Planning & Building Control £1,308

Statutory council planning portal application fee (£328) + Local Authority (HBC) building control inspection fees.

Include Phase 3 Site Supervision?

Have ES Architecture administer the building contract on site.

Specifications In Detail

Understanding Our Three Finish Packages

We believe in complete transparency. Review what is included in each specification tier and why we recommend the Premium Finish as our family home standard.

Base Finish

Standard materials meeting UK Building Regulations. Suitable for rental properties.

New Build£2,350/m²

Renovation£980/m²

Kitchen£6,500

Bathroom£3,800

Standard double-glazed uPVC windows & doors
Building-grade PIR insulation panels
Laminate timber flooring (£25/m² baseline)
Trade kitchen with laminate worktops
Standard white sanitaryware with chrome taps
White plastic electrical faceplates & pendants

★ Recommended Baseline

Premium Finish

Our middle service benchmark for family homes. High-end fixtures and bespoke joinery.

New Build£2,950/m²

Renovation£1,480/m²

Kitchen£14,500

Bathroom£6,800

Slim-profile aluminium double glazing & bi-folds
High acoustic & thermal building envelope
Engineered oak flooring (£65/m² baseline)
Bespoke shaker/handleless kitchen with Quartz stone
Wall-hung sanitaryware & walk-in wet-room showers
Brushed metal electrical faceplates & LED downlights
Skim-coat plastering with premium washable paints

Luxury Finish

The ultimate bespoke architectural masterpiece with artisan finishes and structural glass.

New Build£3,950/m²

Renovation£2,200/m²

Kitchen£29,000

Bathroom£14,000

Triple-glazed structural frameless glass & oversized pivot doors
Passive-house thermal standard & acoustic isolation
Solid herringbone hardwood & natural marble stone
Fully custom timber chef's kitchen with book-matched slabs
Marble master en-suites, steam room & bespoke brassware
Full smart home integration (Control4/Lutron) & AV systems
Venetian polished plaster & microcement finishes

Ready to Take the Next Step?

Discuss your project feasibility directly with Principal Architect Edward Seaman. We review your site, local council planning constraints, and construction budget.

📅 Book Strategy Consultation