Decide early, build once: what it costs to change your plans once building starts

Changing your plans once building work has started costs the new work plus everything the change undoes: work taken out and rebuilt, materials ordered again, made-to-measure items made twice, trades rebooked and sometimes approvals amended. There is no set price, because it depends on how much has been built around the decision. The same change made on a drawing usually costs only a redraw.

Some changes are unavoidable, and some are worth paying for. The aim is to make each decision at the moment it is cheapest, which is usually earlier than it feels necessary. For the wider budget, from surveys to VAT, see our guide to keeping a London renovation on budget.

A variation is any change to the work after the price is agreed: something added, something left out, or the same thing done another way. Choosing a different tile from the one specified is a variation; so is moving a doorway or adding sockets to the kitchen island.

Some are unavoidable, such as repairs nobody could see until the building was opened up. Many others are decisions that could have been made on paper and were made on site instead. How each change should be instructed, priced and approved in writing is set out in our budget guide. This guide is about timing.

Each variation is priced and added to the contract sum, or taken from it. A good contract says how, for example from the rates in the priced quote. Agree the price in writing before the work is done: if nothing fixes it, the Consumer Rights Act 2015 means you pay a reasonable price, and what is reasonable is a question of fact, argued once the work is done.

Once building has started, a change can carry five kinds of cost:

• The new work: the labour and materials for what you now want.
• Undoing: taking out what was built and making good around it.
• Materials: anything bought or made to measure that no longer fits, and the new order.
• Time: the days the site waits and the trades whose dates move, while the scaffold stays up and the site is still run.
• Design and approvals: redrawing, the engineer's checks and, sometimes, the council or your neighbours again.

Made on a drawing before the price is agreed, the same change carries only the redraw.

Take one decision: where the sink goes in a new kitchen extension.

1. 01On the first design options, moving it is a few lines on a drawing.
2. 02After the building regulations drawings, the drainage drawn under the floor is revised too.
3. 03Once the floor slab is laid over the waste pipe, moving the sink means breaking out the floor, re-running the pipe and relaying the insulation, any underfloor heating and the screed.
4. 04Once the kitchen is made to the drawing, the units around the sink may have to be made again.

The sink never changed. What changed is how much had been built around the decision.

The same applies to long lead items. Windows and doors, structural steel and bespoke kitchens are made to the sizes on the drawings and ordered well before they are needed. A change after the order means a new order and a new wait, and an item already made may still have to be paid for.

Trades run in sequence too: the electrician and plumber before the plasterer, the plasterer before the tiler and decorator. When one waits for a decision, the trades after it wait, or move to another job and return when they are free.

Planning permission approves a set of drawings. To change what it approved, such as a larger window, a different roof or another facing material, there are three routes. Their fees are set by law, and for a house from 1 April 2026 the Planning Portal's fee schedule gives:

• A non-material amendment: for a small change, £46. The council has 28 days to decide, or longer if agreed in writing. There is no legal definition of non-material: it depends on the scheme as a whole.
• Varying a condition: where the permission has a condition listing the approved drawings, a larger change can be made by varying it, £89. The result is a new permission in its own right.
• A new application: for a fundamental or substantial change, £548, and the wait starts again: the council has 8 weeks to decide a householder application once it is validated.

Draft regulations would raise these to £53, £112 and £575 from 8 December 2026, if Parliament approves.

Your neighbours may need telling again too. The Party Wall etc. Act 1996 requires notice before the work it covers: at least two months for work to a party wall, and at least one month for a new wall at the boundary or digging near a neighbour's foundations. A change that adds work the notices did not cover, such as a new beam bearing on the party wall, needs a notice of its own, with its own notice period. See party wall agreements explained.

A structural change also needs new calculations from the engineer and revised drawings for building control, whose inspectors check the work at stages through the build.

Every decision has a last cheap moment, after which changing it means undoing something. Five matter most:

1. 01Before the planning application: the outside.
2. 02Before the building regulations drawings: the structure, the layout and the services.
3. 03Before the build starts: the specification and anything with a long lead time.
4. 04Before first fix: every socket, switch, light and pipe.
5. 05Before second fix: the fittings, finishes and colours.

For the order the trades come in once work starts, see what order to renovate a house in.

Planning drawings show what the council will judge, so settle these before the application goes in:

• The footprint, the height and the roof: flat, pitched or glazed.
• The openings: the size and position of windows, doors and rooflights.
• The materials: brick, render or cladding, and the window frames.
• The broad layout, since it decides where the windows need to be.

This is the cheapest moment in the project to change your mind. MVV's initial drawings cover the measured survey, design options, the planning drawings and making the application: £800 to £1,120 for a single-storey extension under 100 m², indicative and ex VAT. See design before you build.

The building regulations drawings, with the engineer's calculations, are what building control checks and the builder prices from. MVV's full design package adds them, with the building control application: £2,000 to £3,000 for the same extension, indicative and ex VAT. Before they are drawn, fix:

• The structure: which walls come out, where the beams sit, how wide the openings are.
• The kitchen layout: the sink, the hob and its extraction, and any island.
• The bathrooms: where the shower, bath and WC sit, which sets where the waste pipes run.
• The heating: radiators, or underfloor heating laid in the floor.
• Large glazing, whose sizes set the steels above it.

The kitchen belongs here, not at the end: its layout decides the drainage, extraction and electrical supplies, all built in long before the units arrive. Our kitchen extensions service designs and builds both.

Every choice still open when the price is agreed goes into the quote as an allowance, or comes back later as a variation. Before work starts, and ideally before the price is agreed, choose:

• Windows and doors, with their frames, glass and colours, so they can be ordered in time.
• The kitchen: units, worktops and appliances, with each appliance's size and position.
• Sanitaryware, taps and tiles: the tile size sets out the shower niche and the valve positions.
• Flooring and its thickness, since floor levels are set early.
• Joinery: wardrobes, shelving and anything built in, drawn and specified.

A design visual helps: moving a niche on a drawing costs far less than moving it once it is tiled.

First fix is the cables, pipes and back boxes run inside walls, floors and ceilings before they are plastered or boarded. After it, moving a socket means cutting into a finished wall, making good and redecorating. Before it, walk the plan room by room and fix:

• Sockets and switches: where, how many and at what height, including in the island and beside the bed.
• Lighting: every downlight, pendant, wall light and lighting strip, and which switch controls which.
• The kitchen: a supply for each appliance, the hob, the oven and the extractor.
• Bathrooms: the shower valve and its height, the niche, the frame for a wall-hung WC, a heated towel rail and any mirror light.
• Television and data: where the screen goes, and the cables behind it.

Our guide to lighting a living room helps plan the circuits.

Second fix is everything fitted after plastering: switch plates and sockets, light fittings, taps and sanitaryware, doors and ironmongery, then the kitchen and the decoration. These choices are mostly about looks, and the cheapest to change, but only until they are ordered and fitted. Fix:

• The light fittings, switch plates and their finishes.
• Internal doors and their handles.
• Paint colours, room by room, woodwork and ceilings included.

Our free second fix calculator lists the materials for these finishes, trade by trade, at four levels of finish, so you can compare them before anything is ordered.

Paint can safely come last, but choosing before the decorators arrive keeps them painting rather than waiting. Our free Colour Room lets you try a colour in a room like yours, in morning or evening light, and keep a mood board, before anyone opens a tin: see how to see your room before you paint.

A specification names each item: the product, the size and the finish. A finishes schedule lists every surface and fitting, room by room. Together they are your decisions on paper, so the builder prices and orders real items, and nobody on site has to guess or wait for an answer. Anything they leave out comes back as an allowance in the quote, which our budget guide explains, or later as a variation.

Keep a contingency as well, for what nobody can know in advance. A change of mind is not what it is for: budget for upgrades separately.

Most of these decisions can start before you speak to anyone:

• The design cost calculator: the surveys, design work and approvals your project is likely to need, with indicative fees, explained in our design costs guide.
• The free estimate: an indicative price range for an extension, loft conversion or refurbishment, so the brief and the budget meet before anything is drawn.
• The second fix calculator: the materials for your finishes, from a best case to a worst case.
• The Colour Room: a colour tried in a room like yours.

All four are free in our Design Studio; see planning your project with free design tools. For what has to be in place before work begins, see before the builders arrive.

Late changes are easier to avoid when the people who draw the decisions also build them. With design and build, the drawings, the price and the programme move together, and a change is priced by the team that will carry it out. See design and build or a designer then a builder and who you need on your side, or book a free consultation.