Six London cash-flow plans for having a child sooner or at 35, planning a second child, or buying before the first birth.
Housing and childcare can collide in your 30s. This scenario follows a newly married dual-income London couple, both 30 with no children, who start in January 2026 with £30,000 invested, aim to retire at 68, and plan through 90. It compares complete cash-flow plans; it does not claim that renting, buying, delaying children, or having fewer children causes a better result by itself.
This page is designed for two London professionals using a broad £80k-£150k combined gross-income range who are weighing four linked questions: one child soon, remaining a dual-income, no-children (DINK) household until 35, adding a second child, or buying before the first birth. It does not calculate take-home pay, so you should check whether its modeled inflows fit your actual income split, tax, National Insurance, pension contributions, student loans, and parental-leave policy.
All amounts are in today's money. Each plan includes a conservative £1,800 monthly State Pension allowance for the couple, not a claim that both partners receive the full new State Pension. Actual entitlement depends on each partner's National Insurance record, and career breaks may change it.
The gross-inflow column averages modeled contributions and other inflows before retirement and before housing, childcare, and family costs. The recurring contributions are a combined investable-portfolio allowance that could include workplace pensions, ISAs, or both; the model does not split them by account or apply account-specific tax treatment. It is not the amount by which the portfolio grows each month; the buy figure also includes the one-off £20,000 family contribution.
Variant
Gross modeled inflows before costs
Capital at age 68 (cumulative interest)
Rent + 1 child soon (Pessimistic)
£1,495/month
£546,989 (£171,509)
Rent + 1 child soon (Base)
£1,495/month
£616,773 (£241,293)
Rent + 1 child soon (Optimistic)
£1,495/month
£741,049 (£365,569)
Rent + 1 child at 35 (Base)
£1,495/month
£685,911 (£310,431)
Rent + 2 children (Base)
£2,155/month
£557,348 (£216,548)
Buy in 2028 + 1 child later (Base)
£2,018/month
£604,440 (£235,360)
Variant
Capital at age 90 (lifetime cumulative interest)
Five-year-buffer monthly budget
Rent + 1 child soon (Pessimistic)
£247,930 (£419,650)
£3,760/month
Rent + 1 child soon (Base)
£294,370 (£604,090)
£4,291/month
Rent + 1 child soon (Optimistic)
£350,184 (£935,904)
£5,274/month
Rent + 1 child at 35 (Base)
£437,045 (£746,765)
£4,598/month
Rent + 2 children (Base)
£279,159 (£548,359)
£4,084/month
Buy in 2028 + 1 child later (Base)
£430,792 (£636,512)
£4,236/month
Every plan stays above zero. In each one, the scheduled monthly outflow—including the later-life care allowance when it applies—is below the model's monthly limit for finishing with five years of spending. The planned outflows are £3,700, £4,200, £5,200, £4,200, £4,000, and £3,800 a month in table order.
The pessimistic, base, and optimistic one-child-soon rows are complete budget cases, not a return-only sensitivity test: they vary both the investment return and planned retirement spending.
The childcare years still require withdrawals in several paths:
One child soon: net portfolio flow is -£600/month from 2028 through 2032.
Remain DINK until 35: the same gross contribution schedule adds £1,600/month from 2028 through 2030, then net flow becomes -£1,200/month from 2031 through 2033 and -£600/month in 2034-2035.
Two children: net flow is +£200/month in 2028-2030, then -£1,200/month in 2031-2033 and -£600/month in 2034-2035. This path deliberately assumes higher gross contributions than the one-child plans.
Buy first: the purchase year removes £82,000 net from the investable pot. Childcare then produces net flow of -£1,000/month in 2030-2032 and -£400/month in 2033-2034.
These are portfolio flows after the entries shown here, not a household budget or take-home-pay forecast. They make the trade-off visible: continuing to schedule contributions does not mean family-year cash flow stays positive.
Compound growth is still back-loaded. In the base one-child-soon plan, modeled interest is about £16,600 at ages 30-39, £30,800 at 40-49, £74,900 at 50-59, and £119,000 at 60-67. Withdrawing during childcare reduces early compounding, even though the portfolio later recovers.
One child soon: first birth in 2028, a £700/month two-bedroom rent step-up, childcare of £1,500/month for three years then £900/month for two years, followed by £220/month of ongoing child costs.
DINK until 35: the base one-child plan's returns, gross contribution schedule, retirement spending, and family-cost amounts are retained, but the housing step-up, birth, childcare, university help, and later housing gift move three years later. This is the timing-only comparison; real fertility, health, and career considerations remain outside the numbers.
Two children: births in 2028 and 2031, overlapping childcare, another £500/month housing step-up for a three-bedroom, and £420/month of later child costs. It also assumes higher contributions and a lower retirement draw, so it is not a same-saving comparison.
Buy first: purchase in 2028 and first child in 2030. This planning illustration uses a £600,000 home, £90,000 deposit (15%), £510,000 mortgage, and 30-year term, all within the supplied London planning ranges. A dated 5.3% Q4 2025 mortgage-rate anchor produces a payment of about £2,835/month; replace it with a live quote. The £465/month owner-cost allowance is a bundled point within the supplied maintenance, service-charge or ground-rent, and buildings-insurance ranges. Compared with £2,500/month baseline rent, those inputs produce the modeled £800/month ownership delta.
The purchase entry is £18,000: the supplied research uses a £10,000 SDLT planning anchor for a £600,000 first-time-buyer purchase, within its stated relief ceiling of £625,000, plus an £8,000 planning allowance for legal work, survey, mortgage fees, moving, and furnishing. This total sits within the research's £13,000-£25,000 SDLT-and-fees range. Check the live HMRC rates and calculator before using the plan, because a 2028 purchase will use the rules then in force. The £20,000 family contribution is separate, so the couple funds £70,000 of the deposit themselves.
The buy path's investable capital bottoms out at about £26,300, so its deposit sprint is demanding: £3,050/month for two years, alongside quarterly investing. Keep a larger emergency reserve if your work, mortgage, or childcare costs are less predictable.
The home itself is not valued here. The buy plan therefore compares investable capital only, not total renter versus homeowner wealth or mortgage equity. Later university support, housing gifts, eldercare, and the care allowance are included in every relevant plan.
Use the guide to changing costs and timing to replace these assumptions with your borough, nursery schedule, mortgage quote, leave policy, and family support.
The model uses out-of-pocket childcare planning amounts, but includes no Child Benefit income. A combined household-income range cannot establish entitlement: the High Income Child Benefit Charge depends on an individual's adjusted net income, including the effect of pension contributions. Add Child Benefit only after checking the current rules and your own income split.
Funded hours and Tax-Free Childcare may reduce costs, but eligibility and usable hours vary. Parental leave, reduced hours, or a pension contribution gap could also make the birth-year squeeze worse; this version does not model an income reduction. A useful stress test is to add a six-month income dip and pause one partner's contributions before relying on the base result.
Prams, cots, car seats, initial clothing, and other one-time baby setup costs are also outside these presets. Add your own birth-year allowance rather than treating the childcare entries as covering them.
The modeled £1,800 monthly State Pension allowance for the couple is a planning assumption, not an entitlement estimate or promise that either partner receives the full amount. Replace it with both partners' forecasts and account for any National Insurance gaps before relying on the retirement result.
The buffer-safe estimates range from about £3,760 to £5,274 a month. Each plan's scheduled retirement and later-care outflow is below its estimate. That margin is scenario-specific, not a universal withdrawal rate, and does not remove return, inflation, tax, longevity, or care-cost risk.
Start with the plan closest to your intended timing, then change:
your actual monthly pension and ISA contributions, especially during parental leave
borough-level rent, childcare, and commuting costs
the purchase price, live mortgage quote, term, deposit, fees, and emergency reserve
Child Benefit or childcare support only after checking individual eligibility
university, future housing, and later-care support
retirement spending and return assumptions together
For a child-free housing comparison, see London couple (32): rent forever or buy by 35?. It provides a related discovery path, while the DINK-until-35 plan above answers the timing question for this same couple.
This is a scenario-planning example, not financial advice. It simplifies taxes, childcare eligibility, parental leave, and housing choices so you can test trade-offs.