The Basement Conversion Process - What To Expect

Step 1: Work out what you want

Basements are genuinely one of the most versatile spaces in a home. Before anything else, it really helps to have a rough sense of what you’re trying to achieve. The most common uses we see are:

• Home gym or yoga studio
• Cinema or entertainment room
• Additional bedroom or guest suite
• Home office or creative studio
• Open-plan kitchen and living space
• Granny annexe or rental income space
• Storage, or utility room

It’s also worth asking whether a basement is the right move for your property at all. If you don’t yet have a loft conversion or an extension, one of those may deliver more space for significantly less cost and complexity. Our team will give you an honest assessment at the survey stage.

Step 2: Get in touch

Call us or use the contact form on our website. You don’t need plans, drawings, or a fixed idea. Just tell us roughly what you’re thinking and where you are, and we’ll arrange a time to come to your home. Basement projects will always start with a proper site survey, because feasibility varies much more from property to property than it does with lofts or extensions.

Step 3: Free site survey and fixed-price quote

One of our experienced surveyors will visit your property to assess the existing basement or substructure, take measurements, look at ceiling height, drainage, access, and the condition of the existing structure. We’ll talk through your requirements and budget in detail, and follow up with a transparent, fixed-price quote. No hidden costs, no surprises mid-build. What we quote is what you pay, unless you change the scope.

Quotes are held for 30 days to give you time to consider and compare. We’re happy to price match, provided any competing quote is as thorough and like-for-like as ours.

Step 4: Design and technical investigations

Basement projects require far more upfront technical work than other conversion types, and this is a good thing. Skipping this stage is how projects run into problems later. Once you’re ready to proceed, we take a deposit, book your start date, and begin the design and investigation phase.

Key things we work through with you:

• Planning permission: most basement conversions require planning permission, particularly where light wells, external staircases, or significant alterations to the building’s appearance are involved. The requirements will vary by local authority and is also affected by whether your property is in a conservation area. We have experience across all London authorities and can manage this process for you
• Party wall agreements: basement excavations almost always affect neighbouring properties and will require a party wall agreement with adjoining owners. Notices must be served in advance and disputes can delay projects if not managed carefully. We guide you through every step
• Structural and technical investigations: before finalising designs, we commission soil analysis, drainage surveys, water table assessment, and arboricultural reports where trees are nearby. Structural engineers and waterproofing specialists are involved from the outset to ensure long-term durability
• Waterproofing strategy: getting waterproofing right is critical, and it needs to be designed in, not added later. We work with specialist waterproofing consultants on every basement project
• Building regulations: all basement conversions must comply with building regulations covering structural integrity, fire safety, ventilation, insulation, and drainage. A building control officer will inspect and sign off the work on completion

Step 5: Construction

Basement construction is typically split into two phases.

Shell and core covers the structural and watertight elements of the build:

• Excavation and, where required, underpinning of existing foundations
• Concrete and steel structural works
• Waterproofing and drainage systems
• Creation of light wells, new access points, and any external alterations

Fit-out covers everything that makes the space liveable:

• Insulation and vapour control layer
• Underfloor heating or radiators
• First fix electrics and plumbing
• Plastering and internal finishes
• Lighting, flooring, and decoration

Most clients will need to vacate the property during at least the excavation phase, as this is the most disruptive part of any basement project. For garden basement conversions, disruption to the main house is significantly reduced. We’ll give you a clear picture of what to expect for your specific project before work starts.

Timelines: a straightforward conversion focused on waterproofing and fit-out can take a few weeks. Projects involving significant excavation or structural changes typically take several months.

Our most complex build to date took 10 months from start to completion. Your dedicated project manager will give you a realistic timeline at the quote stage and keep you updated throughout.

Step 6: Finishing and handover

Once the shell and core works are complete and signed off, we’ll leave you to manage the next phase yourself (if you’ve just opted for the Shell & Core package).

Depending on what you’ve opted for, the finishing stage looks quite different. A full turnkey finish means the space is completely ready to furnish and move into. A whitebox finish means everything is built and plastered, and you take over the decorating and furnishing yourself. If you’ve chosen shell and core only, the fit-out is a separate phase you can return to when you’re ready.

A few things that make a real difference to how a basement feels once finished:

• Natural light: even if you have no windows, options like light wells, glass doors, walk-on glazing at ground level, and light tubes can bring surprising amounts of natural light into the space
• Lighting design: layering spotlights, wall lights, and softer ambient lighting makes a basement feel warm and considered rather than clinical. Recessed lighting is particularly useful where ceiling height is limited
• Ventilation: often overlooked but critical. Good airflow through windows, extractor fans, or mechanical ventilation units keeps the space fresh and controls moisture levels
• Heating: underfloor heating is the most popular choice for basements, as it distributes heat evenly and works well with waterproofing systems

Before handover we’ll do a full walkthrough with you to make sure everything meets the standard you were promised. All our basement projects come with a ten-year structural warranty.

Basement Conversion FAQs

What types of basement conversion are there?
There are three main types that we offer. A half basement extends beneath the original footprint of the house and suits kitchens, living rooms, gyms, or bedrooms (from £150,000 + VAT). A garden basement is built under the garden rather than the house, causing less disruption to the main property (£150,000-£200,000 + VAT). A full basement extends beneath the entire footprint including any extensions, providing maximum space and suit for larger facilities like swimming pools or leisure suites (£400,000-£600,000 + VAT).

Do I need planning permission for a basement conversion?
Most basement conversions require planning permission, particularly where light wells or external alterations are involved. This is different from loft conversions and extensions, many of which fall under permitted development. Requirements vary by borough and conservation area status. We’ll assess your specific property and manage the planning process on your behalf.

Will I need a party wall agreement?
Almost certainly, yes. Basement excavation virtually always affects neighbouring foundations and will require a party wall agreement with adjoining owners. This is a legal requirement and notices must be served before work begins. We manage this process as part of our service.

Do I need to move out during the build?
For most basement projects, particularly those involving excavation and underpinning, we recommend vacating the property at least during the early structural phase. Garden basement conversions are an exception, as the main house is largely unaffected. We’ll give you a clear and honest picture of what to expect for your project before you commit.

How long does a basement conversion take?
It depends on the scope. A simpler conversion focused on waterproofing and fit-out can take a few weeks. Projects involving excavation and structural work typically take three to six months. Complex full basement projects can take up to twelve months or more. We’ll give you a realistic timeline at the survey stage and keep you updated throughout.

How much does a basement conversion cost?
Costs vary significantly depending on the type, size, and specification. As a guide: half basement and garden basement conversions from £150,000 + VAT; and full basement conversions from £400,000 + VAT. All quotes from SimplyEasy Refurbs are fixed-price and transparent. We recommend you budgeting an additional 10% contingency for design changes or unforeseen conditions during excavation.

What is a shell and core conversion?
A shell and core basement delivers the structural, waterproofed, and drainage-complete space, ready for fit-out. It includes excavation, concrete and steel structures, and waterproofing systems, but not internal finishes. Some clients choose shell and core as a first phase, fitting out later. Others go straight to a full turnkey finish. We’re flexible on either approach and happy to be guided by your needs and budget.