Basement Conversion Design Services

Space Below Ground, Designed Properly. Sussex & Surrey.

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Why Sussex Homeowners Trust ArchEvolve With Basement Conversions

A basement conversion involves more structural, waterproofing and Building Regulations complexity than any other domestic project type. ArchEvolve is MCIAT-chartered with 15 years of experience.

      
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   Jonathan Gilbert 
 
 
 
 Trustindex verifies that the original source of the review is Google. I would highly recommend Alex and his company. He has amazing suggestions as well as taking into account our preferences and ideas too. He is easily accessible and has been so helpful in answering all our many questions! Looking forward to seeing our vision come to life!
  
 
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   James Riley 
 
 
 
 Trustindex verifies that the original source of the review is Google. Alex was really helpful throughout the whole process. This was our first ever project like this and he was on hand to answer MANY questions!
  
 
      Posted on Google  
   
   Nick Rutter 
 
 
 
 Trustindex verifies that the original source of the review is Google. Alex was so helpful from start to finish, great communication throughout. Would strongly recommend and will definitely use their services again in the future!
  
 
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   arron tully 
 
 
 
 Trustindex verifies that the original source of the review is Google. Alex at Arch Evolve assisted with obtaining planning permission for a couple of extensions that we were intending on doing. Very professional and offered allot of very helpful advice. Very Happy Thank you.
  
 
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   Marcus Eaton 
 
 
 
 Trustindex verifies that the original source of the review is Google. I have worked with Alex on many projects over the years and he has always been brilliant! Looking forward to working together again soon. Very professional and smooth process throughout the whole process.
  
 
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   Charlotte Morley 
 
 
 
 Trustindex verifies that the original source of the review is Google. Archevolve has completed a number of plans for us due to changing my mind regularly. They have been consistently patient & listened to my requirements at all time. Alex has then been on hand for our builders if required. Fantastic company & highly recommended
  
 
      Posted on Google  
   
   Peter Proto 
 
 
 
 Trustindex verifies that the original source of the review is Google. Used Alex for our large project, loft conversion, side garage and rear extension. Unbelievable service from start to finish. We are over the moon with the finish. Thank you so much!!
  
 
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   Tony Maio 
 
 
 
 Trustindex verifies that the original source of the review is Google. Great service, they had an excellent understanding of my requirements. They applied great attention to detail, with a  fast and very efficient service together very reasonable costs. I highly recommend this firm.

Our services

Basement Conversions Alongside Related Services

Basement conversions are frequently designed alongside a full house refurbishment, and where the property is listed, are coordinated through our listed building design service. For a technical overview of what is involved, see our Building Regulations checklist.

Completed rear extension at Oldfield Crescent

17 services · Sussex & Surrey

Residential

Extensions, conversions and whole-home projects for homeowners, with site visits across West Sussex, East Sussex and Surrey.

Extensions

House extensions Single storey extensions Double storey extensions Rear extensions Side return extensions Wrap around extensions Kitchen extensions Glass extensions

Conversions

Loft conversions Garage conversions Basement conversions Barn conversions

Whole home

House refurbishment Interior design Listed building design Self-build design Sustainable design

20 services · Nationwide

Developers & Commercial

Planning and technical design for developers and commercial clients across England.

Developer services

Design for developers Planning for developers Technical design for housing

Site & feasibility

Housing development feasibility Development site appraisals Residential masterplanning

Conversions

Commercial conversions Office to residential Flat conversions

Development types

New build schemes Residential developments Blocks of flats Infill housing Brownfield sites Affordable housing Mixed-use developments Care homes Student accommodation HMOs Rooftop developments

3D visualisation of a rear extension design

15 services · Homeowners and developers

Planning & Drawings

From the first feasibility check to construction drawings, in the order a project needs them.

01Feasibility & survey

Feasibility studies Measured building surveys Pre-application advice 3D visualisations

02Planning

Planning applications Planning drawings Permitted development Design and access statements Outline planning Reserved matters Discharge of conditions

03Technical & construction

Building regulations drawings Construction drawings Technical design for housing Contract administration

Basement Conversions: Where Structure and Waterproofing Cannot Be Compromised

Converting or digging out a basement carries structural risk that other domestic projects simply do not. Underpinning, waterproofing design and structural engineering are not optional extras, they determine whether the finished space is safe, dry and mortgageable.

Common challenges

Waterproofing specified as an afterthought rather than designed in
Underpinning scope underestimated at the quoting stage
Ceiling heights that do not meet habitable room standards
Fire escape and means of access requirements overlooked
Neighbouring property impact not properly assessed
Contractors without basement-specific experience pricing the work

How ArchEvolve solves them

Waterproofing designed to current British Standard as a core part of the scheme
Underpinning and structural scope assessed accurately before fees are agreed
Ceiling heights confirmed against Building Regulations from the outset
Fire safety and escape strategy designed in from concept stage
Party wall and neighbouring impact assessed early in the process
Party wall and neighbouring impact assessed early in the process

How an ArchEvolve Project Works

Every project follows a clear, stage-by-stage process aligned with the RIBA Plan of Work framework.

Initial Consultation

We discuss your brief, site and budget for your basement conversion project, along with any planning constraints specific to your council. Free, no obligation.

Feasibility Assessment

We assess planning viability and site constraints before any design fees are committed. This includes a structural and ground condition assessment to establish whether underpinning or a full dig-out is required.

Concept Design

We develop the basement conversion design in 3D and produce Enscape renders for your review before planning is prepared.

Planning Application

We prepare and submit the full application to your local planning authority and manage it through to decision.

Building Regulations

We produce the full Building Regulations package covering structure, thermal performance, drainage and fire safety. Basement conversions require detailed structural and waterproofing design, which we coordinate directly with a structural engineer and waterproofing specialist.

Construction Drawings

We deliver the complete technical drawing set your contractor needs to build your basement conversion accurately on site.

Construction Support

We remain available during the build to respond to queries and issue drawing updates where required.

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Areas we cover

Proudly Serving Homeowners Across Sussex and Surrey

Residential projects across West Sussex, East Sussex and Surrey, with site visits and local planning knowledge. Developer and commercial projects are delivered across England.

3Counties

14Towns

UKDevelopers

Town not listed? Ask us

West Sussex11 towns

Storrington Worthing Horsham Chichester Crawley Littlehampton Bognor Regis Burgess Hill Steyning Findon Hassocks

East Sussex2 towns

Brighton Eastbourne

Surrey1 town

Guildford

Developer or commercial project outside Sussex?

We work across England →

A Different Standard of Design for Basement Conversions in Sussex and Surrey

Most practices in the region still work in 2D CAD. ArchEvolve designs basement conversion projects in Revit and renders in Enscape, so you see the finished space before planning is submitted.

3D Visualisations Included

Every basement conversion project is modelled in Revit and rendered in Enscape. You see accurate light, materials and spatial quality before a planning application goes in.

Faster Planning Submissions

Drawings produced in a coordinated 3D environment are more complete and less prone to officer queries, which keeps your basement conversion timeline on track.

Local Planning Knowledge

ArchEvolve has submitted basement conversion applications across West Sussex, East Sussex and Surrey planning authorities. Your submission is prepared for the specific council from day one.

End-to-End Service

From feasibility through to construction drawings, Alex Page manages every stage of your basement conversion project personally. No handoffs, no junior staff taking over mid-project.

Basement Conversion Design Across Sussex and Surrey

ArchEvolve designs basement conversions and dig-outs for homeowners across West Sussex, East Sussex and Surrey.

Converting an existing cellar internally generally does not require planning permission, but a full basement dig-out that increases floor area, or one that requires a lightwell affecting the external appearance, usually does. ArchEvolve confirms your specific position at the free initial consultation.

Yes, significantly. Converting an existing cellar involves waterproofing and finishing an existing space, while a dig-out involves excavating below an existing building, which requires far more extensive structural engineering and underpinning.

Waterproofing is designed to current British Standard as a core part of the scheme, typically combining structural waterproofing with a cavity drainage membrane system, rather than relying on a single method alone.

Basement dig-outs can affect neighbouring foundations, which is why party wall matters and structural impact assessment are addressed early in the design process, before planning or Building Regulations applications are submitted.

Costs vary enormously depending on whether it is a conversion of an existing cellar or a full dig-out, and on ground conditions. ArchEvolve provides a fee estimate specific to your property following the feasibility assessment.

Yes, provided ceiling heights, natural light or borrowed light, fire escape provisions and ventilation meet current Building Regulations for habitable rooms, all of which ArchEvolve designs in from the outset.

ArchEvolve covers West Sussex, East Sussex and Surrey, including Worthing, Horsham, Chichester, Crawley, Guildford and Brighton, with a free initial consultation offered throughout the region.

Frequently Asked Questions

Basement conversion guides

Guides on Basement Conversions

Costs, planning, building regulations, waterproofing and the dig-out process, explained for homeowners before the first drawing.

Finished basement family room under a Victorian terrace, lit by a lightwell with glazed doors

GuideBasement Conversion Cost UK (2026): Cellar vs Dig-OutWhat a basement conversion costs in the UK in 2026, from £1,275 per m² for an existing cellar to £7,500 per m² for a new dig, with line items, fees, VAT and a worked example.Read the guide

Bright converted cellar family room under a Victorian terraced house, with daylight falling from a rear lightwell

Planning and regulationsDo You Need Planning Permission for a Basement Conversion?Converting an existing cellar is unlikely to need planning permission in England, but digging a new basement, adding a lightwell or creating a separate flat usually does. Here is where the line falls.Read the guide

Converted basement family room under a Victorian terrace with an escape window opening into a lightwell and a protected stair

Planning and regulationsBuilding Regulations for Basement Conversions: What AppliesConverting a basement into living space needs building regulations approval. This 15-point checklist covers structure, waterproofing, fire escape, ventilation, drainage, stairs, insulation, electrics and sign-off.Read the guide

Cellar under a Victorian terrace part-way through conversion, with cavity drain membrane fixed to the brick walls and a perimeter drainage channel in the floor

GuideBasement Waterproofing Systems Explained: Type A, B and CBS 8102 sorts basement waterproofing into Type A tanking, Type B concrete and Type C cavity drain systems. Here is how each works, the grades a habitable room needs and how designers choose.Read the guide

Basement dig-out under a Victorian terraced house, with concrete underpinning bays, steel props and an excavated floor in natural light

GuideDigging Out a Basement Under an Existing House: The ProcessThe nine steps of digging out a basement under an existing house in England, from survey and ground investigation to party wall notices, underpinning, waterproofing and sign-off, with typical durations.Read the guide

Converted cellar family room under a Victorian terrace, with daylight from a rear lightwell and pale plastered walls

GuideCellar Conversion Ideas: 12 Ways to Use Space Below GroundTwelve practical cellar conversion ideas, from wine cellars and utility rooms to guest suites and self-contained flats, with the headroom, daylight, escape and waterproofing each one needs.Read the guide

Start Your Basement Conversion Design Today

Free initial consultation for homeowners across Sussex and Surrey. Get in touch to discuss your basement.