7 mistakes homeowners make when renovating, and how to avoid them

Renovation · Before you start

Seven renovation mistakes we see on London houses, drawn from a tender review and our own projects, with what each one costs and how to avoid it.

Last updated: 3 November 2026 · Reviewed by Graham Sproul RIBA, Design & Operations Director

The examples below come from AURA’s projects and from a tender pack we reviewed in September 2026.

The seven renovation mistakes at a glance

Each mistake is explained below, with what we’ve seen on real projects.

Mistake	What it costs you	How to avoid it
1. Treating design as optional	Items priced later as variations	Drawings and a specification for everything the builder prices
2. No contingency	Hidden defects paid from the budget for finishes	A healthy contingency, carried in the tender as well
3. Tendering on unfinished drawings	Prices built on guesses	A tender pack whose documents agree
4. Comparing unlike quotes	Choosing a builder who looks cheaper	Returns adjusted to the same scope before choosing
5. No written contract	No agreed way to price changes	A JCT contract signed before work starts
6. Neighbours and surveys left late	A delayed start	Party wall notices and surveys planned with the design
7. Late finishes and services	Delays waiting for decisions	Layouts and schedules drawn in detailed design

1. Treating the design as an optional cost

A builder can only price what is shown, and anything left to “sort out on site” is priced later, as a variation, when you have no competing quote to compare it with. You don’t need an architect for a renovation by law; our comparison of architect vs design and build sets out when each route fits.

You also don’t have to buy every service at once. AURA’s process guide treats the services as a list to choose from, and we ask for written approval before moving to the next work stage, so the design can stop at the point your project needs.

2. A budget with no contingency

At Chestnut Grove in Balham, the previous builders had built over a Thames Water drain and hadn’t put adequate foundations under the party wall with the neighbours; putting that right cost the clients over £5,000 and two weeks on the build programme.

Keep a healthy contingency, and check the builder’s tender carries provisional sums or a contingency line for the unknowns. In the tender pack we reviewed in September 2026, neither was included. Our guide to renovation cost per m² in London sets out what each level of work typically costs.

3. Tendering on drawings that aren’t finished

Builders price what’s on the drawings. If the drawings are marked “Preliminary”, or they disagree with the specification, each builder fills the gaps differently and the prices can’t be compared.

What was in the pack	The gap	What it risks on site
A floor lowered by about 400 mm	The staircase priced as “1 Item”, with no stair drawing	Called the “biggest single variation risk” in the review: lowering a floor can mean rebuilding the existing stair
Foundation depth	Stated three ways: 1.0 m in the specification, 2.0 m by the engineer, 0.9 m in the bill of quantities	Builders price different foundations, and the quote priced on the shallowest looks cheapest
Basement waterproofing	A Type C cavity drain with no sump, pumps, alarm or discharge route	Described in the review as the most common basement variation
Heating	The specification says a gas combi boiler; the drawings and bill of quantities say an air source heat pump	Two very different prices for the same line
Drawing status	Drawings marked “Preliminary”	Builders pricing a design that is still changing

A lowered floor is a good example of why the drawings matter. Lowering a floor by around 400 mm can mean the existing stair no longer lands, so it has to be redrawn and rebuilt, and the stair needs 2 m of headroom over it. The Building Regulations set no minimum room height, so the room itself can be designed to suit, but a floor or courtyard lowered below the level of the sewer needs a pump or an anti-flood valve. Each of those needs its own line in the tender.

The fix is a tender pack in which every document says the same thing, issued at the right status. Our home renovation process sets out where tendering sits among the other stages. Basement work carries its own version of these risks, covered in our basement conversion guide.

4. Comparing quotes that don’t compare

In the pack we reviewed, the preliminaries (the builder’s site set-up, supervision, welfare and insurances) were left blank, so the returns couldn’t be compared, and a second phase of work was excluded on the drawings but priced in the bill of quantities. At AURA we tender to up to four vetted builders, and the returns are compared line by line before anyone is chosen.

When the returns come back, list every exclusion and qualification side by side. Where one builder has left something out, ask both builders to price it in writing, so the comparison rests on their own figures. The same goes for provisional sums: if one tender carries a larger allowance for an unknown item, it will look dearer for a reason that has nothing to do with the builder’s rates.

Worked example: putting two tenders on the same footing

Two builders return prices for the same renovation. Builder A prices everything, including a second phase of work. Builder B leaves the second phase out, and Builder A prices that phase at £12,000. The figures are illustrative.

Line	Builder A	Builder B
Tender total as returned	£184,000	£174,000
Second phase, excluded by Builder B (priced by A)	Included	+ £12,000
Adjusted total	£184,000	£174,000 + £12,000 = £186,000
Difference	£186,000 − £184,000 = £2,000 in Builder A’s favour

Builder B’s return looked £10,000 cheaper. Once the missing phase is added at Builder A’s rate, Builder B is £2,000 dearer. Builder B may price that phase differently when asked, so the next step is a written query to both builders before either is chosen.

5. Starting without a written contract

Our usual building contract for house projects is JCT Minor Works with contractor’s design, now in its 2024 edition, with JCT Intermediate with contractor’s design on larger jobs. The contract fixes the price and sets out how variations are agreed and paid. Our JCT contracts require the contractor to carry public liability insurance of at least £5 million.

Once work starts, ask for every variation to be instructed and priced in writing before it is carried out. That keeps the final account to figures both sides have already agreed.

Insurance for the existing house and the new work is set out in the contract too, and who insures what is agreed job by job. Our guide to house extension insurance explains what to check with your own insurer before work starts.

6. Leaving the neighbours and the surveys too late

Party wall notices need the engineer’s foundation design, so they can only be served once the structural drawings exist, and you should allow at least two months. Our proposals tell clients to allow about £1,000 per neighbour for party wall costs. Removing a chimney breast on a shared stack is a party wall matter, and we’ve seen neighbours raise party wall concerns as planning objections. Our party wall guide covers the notices and awards.

Surveys belong at the start as well. The pack we reviewed had no asbestos refurbishment and demolition survey, which identifies asbestos in the parts of the house being worked on before anyone cuts into them.

7. Deciding finishes and services too late

Decisions left until the builder asks hold up the trades waiting for them. On AURA projects the detailed design stage includes the heating and electrical layouts, the ceiling and lighting plans, finishes details and window and door schedules, so the electrician and the joiner work to drawings. Our guide to lighting design shows how a lighting plan fits into that stage.

Period houses need extra care here. Internal wall insulation on a solid wall needs a specific detail where the floor joists meet the wall, or condensation can rot the joist ends; the same tender review flagged it. Our guide to renovating a Victorian or Edwardian house covers the wider decisions on an older house.

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