When you are planning an extension, the builder’s quotation is only one part of the budget. Surveys, design work, approvals and specialist advice can all need paying for before construction begins.
Not every home needs every service. The useful starting point is to identify what your particular property and proposal require, then separate the design fees, third-party charges and construction allowance.
This guide explains the main costs to consider for a residential project in Worthing and West Sussex. Downland Studio’s published fees are starting prices; your itemised proposal confirms the agreed scope and fee.
1. Survey, design and technical drawings
A detailed digital measured survey records your existing home. We use specialist measuring technology, check important dimensions with precision laser measurement where required, and interpret the information to produce the existing Revit model. Design and planning drawings develop the proposal, while Building Regulations and technical design work through how it will be built.
Measured Building Survey — from £400. Digital measured information and our existing model and drawings for the agreed scope, with a PDF measured floor plan supplied through your Client Portal.
Design & Planning — from £800. Design development and architectural drawings for the appropriate planning route, with submission support where included in your appointment.
Building Regulations — from £1,850. Detailed architectural drawings and relevant specifications for Building Control review and construction, coordinating separately appointed specialists’ information as appropriate.
Complete Design & Technical Package — from £2,650. The digital measured survey, design and planning stage, and technical drawing package. Where the qualifying full package is instructed, the measured survey fee is credited against the final Building Regulations invoice.
Loft Conversion Package — from £2,250. A project-specific design and technical package for roof-space alterations; the scope depends on the roof, accommodation and proposed work.
No VAT to add. Downland Studio is not currently VAT registered, so VAT is not added to the fees shown. Third-party charges have their own tax treatment.
Explore the full fee guide and architectural fees calculator →
2. Specialist information and additional approvals
Structural engineering
Extensions, loft conversions and structural alterations commonly need an engineer to design or assess the relevant structure. The engineer is normally appointed and paid separately by you. Ask for a quotation based on the actual work; complexity, ground conditions and the information required all affect their fee.
Downland Studio can coordinate the engineer’s information with the architectural drawings. Each specialist remains responsible for their own professional work.
Building over or near a sewer
Work over or close to a public sewer may need the water company’s approval. The route depends on the sewer and proposed construction; Southern Water offers self-declaration and formal approval routes for suitable projects.
Downland Studio’s Southern Water Build Over Application service starts from £295 for the agreed architectural information and application support. Water-company charges, surveys and any separately required structural design are additional. Check the current requirements and charges for your site before relying on a budget allowance.
Southern Water building and development guidance →
Party wall notices
Work to a shared wall, building at a boundary, or certain excavations near neighbouring foundations can fall within the Party Wall etc. Act 1996. The detail matters: proximity alone does not settle whether a notice is needed.
Downland Studio’s Party Wall Notice Pack starts from £500 per adjoining owner. This is notice preparation, not a Party Wall Surveyor appointment or an Award. If an owner dissents or an Award is required, a suitably qualified surveyor must be appointed separately, with their fee agreed directly.
Principal Designer appointments
Principal Designer roles under the Building Regulations and the Construction (Design and Management) Regulations 2015 are separate. Building Regulations duties concern the design’s compliance with building requirements; CDM duties concern health and safety. Appointment requirements depend on the project and the designers or contractors involved.
Downland Studio’s published £450 Principal Designer Duty Holder service relates to the Building Regulations role, subject to an agreed appointment and scope. It does not automatically include a CDM appointment, construction supervision or responsibility for another specialist’s work.
Government guidance on Building Regulations duties →
3. Application and Building Control charges
Planning application fees depend on the application category and the fee rules in force when you apply. Budget separately for any Planning Portal service charge and any additional reports. Check the current charge rather than relying on an older householder fee quoted online.
Check the government’s planning fee guidance →
Building Control charges also vary with the proposed work and appointed body. Ask for a project-specific quotation covering the application and relevant inspections. The contractor remains responsible for constructing the works correctly; Building Control carries out its relevant inspections, and completion certification follows satisfactory completion and the required inspections and information.
4. Services needed only on some projects
Your proposal may also need work relating to planning conditions, changes to an approved design, a Lawful Development Certificate, heritage matters or a highways crossover. These are separate tasks with their own scope and, where relevant, authority charges.
For example, Downland Studio’s discharge of planning conditions, minor planning amendment and dropped-kerb drawing services each start from £295. A Lawful Development Certificate service starts from £400. The current fee guide explains the services; your proposal confirms what is actually included.
Allow for the support you want during construction
If you would like formal help administering the building contract, allow separately for optional Contract Administration. This can cover payment assessments, variations, instructions and contractual completion within an agreed appointment. It is a separate fee from design and technical drawings, confirmed against the project scope and construction value. Lighter, flexible Project Support may suit a different need.
Build a budget around your project
Before committing to a construction budget, allow for:
The agreed survey, design and technical drawing stages.
Separately appointed engineers or other specialists.
Planning, Building Control and any water-company charges.
Project-specific notices, reports and additional approvals.
A realistic construction allowance and contingency.
A single pre-construction total cannot cover every home. Our extension budget calculator helps you explore an initial allowance, and a tailored proposal makes the design scope clear before work begins.
Explore the extension budget calculator →
Tell us about your home and the changes you are considering →