Retentions, down-valuations and warranty rules explained by a whole-of-market broker
New build mortgages work the same way as any other residential mortgage on the surface — same affordability checks, same credit assessment, same LTV bands.
What’s different is everything that happens around the mortgage: how the property is valued before it even exists, what a lender will and won’t release money for, and what happens years later if the original purchase came with a Help to Buy equity loan attached.
Most of the problems buyers run into with new build finance aren’t about qualifying for the mortgage itself — they’re about these surrounding mechanics catching people off guard partway through the process.
How New Build Mortgages Differ From Buying an Existing Home
An existing property has a track record: recent sales of similar homes nearby, a physical structure a surveyor can walk through and assess against decades of comparable data. A new build often has neither.
The surveyor is valuing a show home, a floor plan, or a part-built shell, then weighing that against a development where nearly every other sale went through the same developer’s incentive schemes — deposit contributions, stamp duty paid, furniture packages thrown in — none of which show up as a discount on the headline price but all of which affect what the property is genuinely worth.
That gap between the price on the reservation form and what an independent valuer is willing to confirm is where most new build mortgage problems start, and it feeds into two issues covered below: retentions and down-valuations.
Mortgage Retentions on New Build Properties
A retention is simply the lender holding back part of the mortgage until something specific gets finished. On a new build, this usually isn’t about a defect the way it would be on an older property — it’s about the development itself not being complete yet.
Roads and footpaths not yet adopted by the local authority, landscaping still to be finished, communal areas or a shared driveway still under construction: any of these can trigger a lender to release most of the funds at completion and hold the rest back until the developer finishes the outstanding work.
The amount held back and the conditions for releasing it vary lender to lender and case to case, so it’s worth asking your broker to check for this specifically before exchange, not after it turns up unannounced in the mortgage offer. Some developers build the likely retention into their build-completion timeline; others treat it as a nuisance to be argued about after the fact.
A lender’s valuation can trigger a retention or a down-valuation
Down-Valuations: When the Lender’s Valuer Disagrees With the Price
A down-valuation happens when the surveyor instructed by the mortgage lender values the property below the price agreed with the developer.
It’s a live risk on new build purchases specifically because of the pricing gap described above — new homes in Great Britain currently sell at roughly a 30% premium over comparable existing properties on average, up from around 23.7% a decade ago, according to research published in August 2026. That premium isn’t evenly spread: it runs as high as 60% in the North East and 55% in Scotland, while London stands alone as the region where new builds actually sell at a discount to resale property, around 10.5% below.
Where the premium is highest, a valuer working from resale comparables has the most room to land below the developer’s asking price. If that happens, the mortgage is offered against the lower figure, and the buyer is left covering the shortfall in cash, renegotiating with the developer, or walking away and losing costs already spent.
This is the single biggest reason it’s worth using a broker and solicitor who’ve handled new build purchases with that specific developer before — they’ll often know whether down-valuations have cropped up repeatedly on that development.
NHBC Buildmark and Why Lenders Require a New Build Warranty
Almost every mainstream lender requires a recognised structural warranty before they’ll mortgage a new build property, and NHBC’s Buildmark is the most common.
It works in two stages: for the first two years, the builder is directly responsible for fixing defects, with NHBC’s own resolution service stepping in if the builder won’t. From year three through year ten, cover shifts to an NHBC insurance policy against physical damage caused by the home not being built to their technical standards — structural issues that wouldn’t normally surface until years after the builder has moved on.
The warranty also protects deposits paid before completion — up to £100,000 — should the developer become insolvent mid-build, and covers repair costs up to £1 million on a standard home. Lenders require it because it cuts their own exposure on a property type with no track record — no warranty generally means no mortgage offer, full stop, regardless of how strong the buyer’s own financial position is.
Help to Buy Equity Loan Repayment: What Happens When It’s Due
Help to Buy — one of the most widely used first-time buyer mortgage schemes for new build purchases — stopped accepting new applicants in 2023. Even so, a large number of homeowners are still living with an active equity loan from a purchase made in the scheme’s final years, and the repayment mechanics catch people out because they’re not what most borrowers expect from a loan.
The first five years are interest-free. From year six, interest starts at 1.75% of the loan amount, then rises each year in line with inflation plus a set margin.
The part that surprises people most: the amount owed isn’t fixed at the amount originally borrowed. It’s recalculated as a percentage of the property’s current market value at the point of repayment. A loan that covered 20% of the purchase price still represents 20% of whatever the home is worth when it’s repaid — so a property that’s gained substantial value means the equity loan has grown in cash terms right along with it, even though nothing else about the loan has changed.
The full loan has to be repaid within 25 years of purchase, or immediately on sale of the property, whichever comes first. Anyone approaching that six-year mark, or planning to sell or remortgage, needs the up-to-date repayment figure confirmed well ahead of time, rather than assuming the original loan amount still applies.
New Build vs Resale: Which Is Easier to Mortgage?
Getting a mortgage approved isn’t harder for one over the other, but new build brings the retention, valuation and warranty issues above into play, plus a purchase timeline that’s often dictated by the developer’s build schedule rather than the buyer’s.
Mortgage offers on most types of mortgage typically last six months, and a delayed completion date can mean reapplying or extending the offer, sometimes at a different rate than originally agreed if current mortgage rates have moved.
Resale purchases don’t carry that build-completion risk, but they don’t come with a ten-year structural warranty either, and older properties can throw up their own retention triggers around damp, roofing or electrics that a new build simply won’t have.
Frequently Asked Questions
What deposit do I need for a new build mortgage?
Deposit requirements are generally the same as for any other property, though some lenders cap new build loan-to-value slightly below their standard maximum. Check this early if you’re planning on a high-LTV purchase — a 100% mortgage is rarely available on new build specifically.
Can a mortgage offer expire before my new build is finished?
Yes — most offers last around six months, and new build completion dates slip regularly. If the build overruns, you might have to ask for more time on the offer or submit a fresh application, and rates may have shifted in the meantime.
Do I need a NHBC warranty specifically, or will any warranty do?
NHBC’s Buildmark is the most widely accepted, but most mainstream lenders will also accept other recognised providers such as Premier Guarantee or LABC Warranty. The cover just needs to satisfy their own requirements.
Will I definitely have a mortgage retention on a new build?
No — many new build completions go through without one. It’s specific to whether parts of the development, such as roads, communal areas or landscaping, are still unfinished at the point your purchase completes.
I still have a Help to Buy loan — does it affect my ability to remortgage?
It doesn’t prevent a remortgage. But if you’re clearing the equity loan along the way, it needs factoring into the new mortgage amount, and the repayment figure is based on a current valuation rather than the original loan amount.
Final Thoughts
New build mortgages aren’t a different product from a lender’s point of view, but the property itself brings risks that don’t apply to an existing home — a valuation gap that can trigger a down-valuation, work that isn’t finished yet that can trigger a retention, and in some cases a Help to Buy loan whose repayment terms are easy to misunderstand years down the line.
None of this makes a new build a bad purchase; it just means the finance side needs a broker who’s dealt with these specific issues before, not just a standard mortgage application.
UK Mortgage Broker is a whole-of-market broker directly authorised and regulated by the Financial Conduct Authority, working with buyers across new build and resale purchases throughout the UK. We can flag likely retention and warranty requirements before you commit, and help fit your application around a developer’s build schedule instead of working against it.
Your home may be repossessed if you do not keep up repayments on your mortgage.
Speak to a whole-of-market specialist before you commit
Call us on 01628 969 500 or email info@uk-mortgagebroker.co.uk to speak to a specialist directly.
Related Pages
- Mortgage Rates UK – current UK mortgage rate ranges by LTV band
- Types of Mortgage UK – fixed, tracker and interest-only options explained
- Mortgage Affordability UK Explained – how lenders decide what you can borrow
- Who Is Eligible for a 100% Mortgage in UK? – no-deposit mortgage eligibility
- Residential Mortgages – whole-of-market residential mortgage advice
- Buy to Let Mortgages – landlord and portfolio mortgage advice